7. Offshore Plankton and Benthos of the Gulf of Mexico

Jacks and pompanos are both ecologically important as predators and prey (Lyczkowski-Shultz et al. 2004). Some species are important in the commercial and recreational fisheries in the Gulf of Mexico and are highly regarded as food (e.g., pompano), game fish (e.g., amberjack), or bait (e.g., blue runner, Caranx crysos) (Ditty et al. 2004; Lyczkowski-Shultz et al. 2004). Larval jacks and pompanos cannot be reliably identified to species; however, they can typically be identified to genus (Lyczkowski-Shultz et al. 2004).

Jacks and pompanos made up more of the total ichthyoplankton catch than any of the other groups selected for analysis (Table 7.1). More larval jacks and pompanos were captured during the fall along the continental shelf, as compared to spring in the open Gulf of Mexico and, while larvae were captured both at the surface and in the water column, the majority of larval jacks and pompanos were captured at the water surface in neuston nets (Table 7.1).

The average abundance of Carangidae larvae collected by neuston net during the spring ranged from 1.6 (2004) to 18.5 (1987) larvae per 10-min tow, and the average abundance of carangids collected by bongo net ranged from 12.3 (2004) to 134 (1986) larvae per 10 m² (Figure 7.14). Carangid average abundance during the fall along the continental shelf ranged from 1.4 (1983) to 23.1 (1999) larvae per 10-min tow for neuston net samples, while bongo net samples ranged from 6.9 (1983) to 98.4 (1999) larvae per 10 m² (Figure 7.15). In general, the average abundance of Carangidae larvae was typically higher along the continental shelf during the fall as compared to the spring in the open Gulf for both gear types. With the exception of 1985 and 1986, the average abundance of larvae for bongo net samples was within a similar range during the spring; however, average carangid larval abundances were highly variable from year to year during the spring for neuston samples and during the fall for both gear types from 1982 through 2007 (Figures 7.14 and 7.15).

During the spring, the majority of larvae captured were jacks (Caranx), while most of the jack and pompano larvae that were obtained during the fall were Atlantic bumper (Chloroscombrus chrysurus) and round scad (Decapterus punctatus) (Table 7.2). Jacks were distributed throughout the open Gulf of Mexico during the spring, as well as throughout most of the continental shelf during the fall (Figure 7.16). While Atlantic bumper larvae were distributed throughout the entire continental shelf during the fall, larvae were sparsely distributed throughout the Gulf during spring plankton surveys (Figure 7.17).

As forage fish, herrings, shads, sardines, and menhadens are abundant coastal pelagic species that constitute an important, if not primary, food source for many predatory game and commercial fishes (Shaw and Drullinger 1990a). Most of the herring, shad, sardine, and menhaden larvae that were captured during the plankton surveys from 1982 through 2007 were captured during the fall along the continental shelf, and similar numbers were taken in both the neuston and bongo nets (Table 7.1). Thirteen taxa are included in this group, with the most larvae being scaled sardines (Harengula jaguana), round herring (Etrumeus teres), and Spanish sardine (Sardinella aurita) in the spring and Atlantic thread herring (Opisthonema oglinum), Spanish sardine, and scaled sardines in the fall (Table 7.2). While scaled sardines and Atlantic thread herring were distributed throughout the continental shelf during the fall, they were sparsely distributed throughout the open Gulf and Florida Shelf in the spring (Figures 7.18 and 7.19).

The Gulf menhaden (Brevoortia patronus) is one of the most abundant pelagic fishes in the northern coastal Gulf of Mexico; it is an exploited marine resource, the principal prey for many important commercial and recreational fish species, as well as marine birds and mammals. As both a planktivore and detritivore, Gulf menhaden are an integral and key component of the Gulf of Mexico ecosystem (Vaughan et al. 2011). However, because adults spawn primarily near the mouth of the Mississippi River during the winter and the plankton surveys were conducted in the spring and fall, only one Gulf menhaden juvenile was collected in the plankton surveys (Table 7.2).

From 1982 through 2007, average clupeid larval abundances were highly variable from year to year during the spring and fall for both gear types (Figures 7.20 and 7.21). For neuston net samples, the average abundance of Clupeidae larvae ranged from 0 (2004) to 68.5 (1983) larvae per 10-min tow in the spring in the open Gulf and from 1.3 (1983) to 97.4 (1993) larvae per 10-min tow during the fall along the continental shelf (Figures 7.20 and 7.21). Average clupeid larval abundance for bongo net samples ranged from 11.7 (1982) to 247.2 (1999) larvae per 10 m² and from 6.7 (1983) to 225.9 (1995) larvae per 10 m² during spring and fall plankton surveys, respectively (Figures 7.20 and 7.21). For both gear types, the average abundance of Clupeidae larvae was typically higher along the continental shelf during the fall, compared to spring in the open Gulf of Mexico.

Dolphinfishes (sometimes referred to as mahi mahi or dorado) are an important commercial and recreational species distributed throughout the tropical and subtropical seas of the world and are highly prized for food (Ditty et al. 1994). They are often associated with Sargassum spp. or other floating objects. One of the fastest growing species in the ocean, dolphinfish, serves as a primary food source for many pelagic predators (Palko et al. 1982). This group includes four taxa (Table 7.2). Most dolphinfish larvae were taken in the spring in the open Gulf of Mexico and at the water surface in neuston nets (Table 7.1). Dolphinfish were fairly well distributed throughout sampling stations during both spring and fall plankton surveys conducted from 1982 through 2007 (Figure 7.22). During the spring, as well as during the fall along the continental shelf, average abundances of dolphinfish larvae occurred at low densities and typically ranged from 1 to 3 larvae per 10-min neuston tow (Figures 7.23 and 7.24). For bongo net samples, the average abundance of coryphaenid larvae generally ranged from 5 to 9 larvae per 10 m² (Figures 7.23 and 7.24). In the spring in the open Gulf of Mexico, the highest average abundance of larval dolphinfish for samples collected by bongo net occurred in 2007, while the highest average abundance occurred in 1998 during the fall along the continental shelf (Figures 7.23 and 7.24).

Billfish, marlin, and sailfish are highly migratory across vast expanses of open ocean; therefore, not much is known about their life histories, especially the larval stages (Tidwell et al. 2007). Billfish support a sport fishery worth hundreds of millions of dollars each year, and as top predators play a critical role in all pelagic ecosystems (Tidwell et al. 2007; Rooker et al. 2012).

Most larval marlin and sailfish were taken in the spring in the open Gulf and at the water’s surface, and seven taxa were included in this group (Tables 7.1 and 7.2). Though they did not occur at all sampling stations, billfish were fairly well represented during spring and fall plankton surveys (Figure 7.25).

Average abundances of billfish larvae for neuston net samples typically ranged from 1 to 4 larvae per 10-min tow during both the spring and fall, indicating similar surface densities in both the open Gulf and continental shelf (Figures 7.26 and 7.27). For neuston net samples from 1982 through 2007, the highest average abundance of billfish larvae occurred in 2003 during spring surveys and in 1998 during fall surveys. The average abundance of billfish larvae was typically higher during the spring on the open Gulf as compared to along the continental shelf during the fall for bongo net samples (Figures 7.19 and 7.20). The highest abundance for all spring and fall bongo net samples, more than 30 larvae per 10 m², occurred in 1986.

The snapper family includes mostly reef-associated species, as well as several deepwater species; due to the excellent quality of its meat, snappers are of significant importance to the commercial and recreational fisheries in the Gulf of Mexico, and many species are overfished (Martinez-Andrade 2003). For example, red snapper (Lutjanus campechanus) is one of the most important food fishes in the Gulf of Mexico, and this fishery, which collapsed in the eastern Gulf of Mexico in the late 1980s, is the most controversial fishery in the U.S. Gulf of Mexico (Johnson et al. 2009; Cowan et al. 2010).

The majority of larval snapper were captured during the fall plankton surveys along the continental shelf, and most were taken in the water column in the bongo nets (Table 7.1). The snapper group includes 12 taxa, with most of the larvae captured during both the spring and fall consisting of the snapper family (Lutjanidae) and vermillion snapper (Rhomboplites aurorubens) (Table 7.2). Both Lutjanidae and vermillion snapper were found along the entire continental shelf during fall plankton surveys, and they were not distributed widely during spring plankton surveys (Figures 7.28 and 7.29).

From 1982 through 2007, the average abundance of lutjanid larvae collected by neuston net during the spring in the open Gulf of Mexico ranged from 0 (1983, 1988, and 2004) to 6.8 (1990) larvae per 10-min tow, while the average abundance of snapper larvae collected by bongo net ranged from 4 (1982) to 22.7 (1986) larvae per 10 m² (Figure 7.30). Snapper larvae average abundance during the fall along the continental shelf ranged from 0 (2006) to 12.4 (1987) larvae per 10-min tow for neuston net samples, and bongo net samples ranged from 5.6 (1983) to 24.2 (2001) larvae per 10 m² (Figure 7.31). For both gear types, the average abundance of snapper larvae was typically higher along the continental shelf during the fall as compared to the spring in the open Gulf (Figures 7.30 and 7.31).

Mullet are ecologically important in the flow of energy through estuarine communities because they are primary consumers that feed on plankton and detritus. In the Gulf of Mexico, mullet typically spawn many miles offshore in deep water (Collins 1985). Mullet are important prey species for many fish and are also important to the recreational and commercial fisheries. As silvery pelagic juveniles, mullet inhabit surface waters of the open ocean for several months before migrating inshore (Lyczkowski-Shultz et al. 2004).

Four taxa are included in this group, with the majority of larvae identified to the genus Mugil (Table 7.2). Most mullet larvae were taken in the spring plankton surveys in the open Gulf of Mexico and were captured in the neuston nets at the surface (Table 7.1). Mullet were found in the open Gulf, as well as in the continental shelf during spring and fall plankton surveys from 1982 through 2007 (Figure 7.32).

Average abundances of larval mullet were highly variable from year to year during spring and fall for both gear types from 1982 through 2007 (Figures 7.33 and 7.34). For neuston net samples, the average abundance ranged from 0.97 (1985) to 23.8 (1999) per 10-min tow in the spring in the open Gulf and from 0 (1983, 2003, and 2006) to 15.3 (1984) larvae per 10-min tow during the fall along the continental shelf (Figures 7.33 and 7.34). Average mugilid larval abundance for bongo net samples ranged from 2.8 (1983) to 24.1 (1986) per 10 m² and from 0 (1983, 1986, 1998, and 2002) to 18.6 (2004) larvae per 10 m² during spring and fall plankton surveys, respectively (Figures 7.33 and 7.34). The average abundance of mullet larvae was typically higher during the spring in the open Gulf, compared to fall along the continental shelf for both gear types.

Members of the Family Sciaenidae (drums and croakers) are an important sport and commercial fishery resource along the U.S. Gulf of Mexico and are perhaps the most prominent group of northern Gulf inshore fishes (Cowan and Shaw 1988). This group includes 15 taxa, with most of the larvae consisting of the drum and croaker family (Sciaenidae), with the kingfish genus (Menticirrhus) in the spring and the kingfish genus and redfish in the fall (Table 7.2). The vast majority of larval drum and croaker were found during fall plankton surveys, with the most in the water column in the bongo net samples (Table 7.1). The drum family (Sciaenidae) larvae were more extensive along the continental shelf during the fall compared to the spring (Figure 7.35); the seasonal distribution of this group was even more dramatic for the redfish (Figure 7.36).

From 1982 through 2007, with the exception of 1991 and 1995, the average abundance of sciaenid larvae collected by neuston net during the spring in the open Gulf of Mexico was fewer than 6 larvae per 10-min tow, and drum and croaker larval abundance averaged fewer than 20 larvae per 10 m² for bongo net samples during the spring, with the exception of 1986, when the average larval abundance was more than 120 larvae per 10 m² (Figure 7.36). Drum and croaker average abundance ranged from 1.3 (2006) to 32.2 (2000) larvae per 10-min tow for neuston net samples, and bongo net samples ranged from 4.8 (1983) to 208 (1988) larvae per 10 m² during the fall (Figure 7.38). For both gear types, the average abundance of sciaenid larvae was typically much higher along the continental shelf during the fall compared to the spring in the open Gulf (Figures 7.37 and 7.38).

Redfish larvae were concentrated higher in the water column during daylight hours than at night in the general area east of the Mississippi Delta and south of the Mississippi barrier island over the East Louisiana–Mississippi–Alabama shelf in September and October 1984 and 1985 (Lyczkowski-Shultz and Steen 1991). In addition, there was no clear relationship between vertical aggregation of red drum larvae and temperature or salinity profiles or microzooplankton prey distribution. Atlantic croaker (Micropogonias undulatus) larvae were found to be least abundant in surface waters at night, and the highest abundances at night were observed at the deepest depths sampled during an investigation conducted in inner-shelf waters off Mississippi during September and October 1984 and 1985 (Comyns and Lyczkowski-Schultz 2004). By midmorning, Atlantic croaker larvae had moved up the water column, and highest abundances were usually found at 5 m (16.4 ft); no consistent pattern was found in the vertical stratification of Atlantic croaker larvae during the midday or afternoon.

Mackerels, tunas (with the exception of Thunnus, discussed in the section below), and bonitos are important recreational and commercial fish species. For example, king mackerel (Scomberomorus cavalla) and Spanish mackerel (Scomberomorus maculatus), which are abundant and highly migratory, are coastal members of the Scombridae family and support large commercial and recreational fisheries (De Vries et al. 1990). This group includes 13 taxa; the majority of larvae for this group consisted of skipjack tuna (Katsuwonus pelamis) and tuna (Auxis) in the spring in the open Gulf of Mexico and little tunny (Euthynnus alletteratus) in the fall along the continental shelf (Table 7.2). Mackerel, tuna, and bonito larvae were typically taken in the fall and in bongo net samples of the water column (Table 7.1). Skipjack tuna were distributed throughout the open Gulf of Mexico during the spring. During the fall, they occurred in locations along the near edge of the continental shelf (Figure 7.39). Little tunny were found at some locations during spring plankton surveys; however, they were densely distributed throughout the entire continental shelf during the fall (Figure 7.40).

Average scombrid larval abundances were highly variable from year to year during the spring and fall (Figures 7.41 and 7.42) for both gear types from 1982 through 2007. The average abundance of Scombridae larvae collected by neuston net during the spring ranged from 1.7 (1988) to 9.7 (1996) per 10-min tow, and the average abundance of scombrids collected by bongo net ranged from 8.7 (1985) to 37.4 (1986) larvae per 10 m² (Figure 7.41). During the fall, scombrid average abundance along the continental shelf ranged from 0 (2003) to 16.5 (1998) larvae per 10-min tow for neuston net samples, while bongo net samples ranged from 4.7 (1983) to 33.4 (1984) larvae per 10 m² (Figure 7.42). The mackerel, tuna, and bonito larvae average abundances were within a similar range during the spring in the open Gulf and during the fall along the continental shelf for both gear types (Figures 7.41 and 7.42).

Atlantic bluefin tuna are large, highly migratory and have been heavily overfished. They spawn in the pelagic Gulf of Mexico during the spring, typically April through June (Teo et al. 2007; Muhling et al. 2010). Adult bluefin tuna have the broadest thermal niche of any of the Scombridae; they make fast, ocean basin-wide scale migrations ranging from cool subpolar foraging grounds to discrete breeding sites in subtropical waters during the spawning season (Teo et al. 2007).

Muhling et al. (2010) used a subset of SEAMAP data from 1982 through 2006 to develop a model of suitable Atlantic bluefin tuna larvae habitat in the northern Gulf of Mexico. The location and size of favorable habitat was highly variable among years. Habitats within the LC, warm-core rings, and cooler waters on the continental shelf were less favorable.

Yellowfin tuna are common in the Gulf of Mexico in pelagic waters and support one of the most valuable commercial fisheries in the Gulf of Mexico (Lang et al. 1994). Lang et al. (1994) determined that significant spawning of yellowfin tuna most likely occurred in the northern Gulf of Mexico in the vicinity of the Mississippi River discharge plume, when 801 larvae were collected during July and September 1987, and enhanced yellowfin tuna larval growth and survival occurred in the plume frontal waters.

Identification of tuna larvae of the genus Thunnus is very difficult (Richards et al. 1990), and because of this most of the larvae for this group were identified only to genus (Table 7.2). Tuna larvae were typically found in the spring in the open Gulf and usually at the surface in the neuston net samples (Table 7.1). They occurred throughout the open Gulf of Mexico during the spring. During the fall, they were typically found at locations near the edge of the continental shelf (Figure 7.43).

From 1982 through 2007, average abundances of tuna larvae were highly variable from year to year during spring and fall for both gear types (Figures 7.44 and 7.45). For neuston net samples, the annual abundances of larval tuna averaged fewer than 8 larvae per 10-min tow in the spring in the open Gulf, with the exception of 1985. During the fall along the continental shelf, average abundance ranged from 0 (2003 and 2006) to 16.6 (1987) larvae per 10-min tow (Figures 7.44 and 7.45). For both spring and fall plankton surveys, annual abundances of larval tuna for bongo net samples were within a similar range and typically averaged fewer than 25 larvae per 10 m² (Figures 7.44 and 7.45).

Twenty-eight taxa are included in this group of seabasses and groupers, with the majority of larvae identified to the seabass family (Table 7.2). Adult grouper are a commercially and recreationally important species that are highly susceptible to overfishing, largely due to their spawning behavior and slow growth (Marancik et al. 2012).

While fairly similar numbers of seabasses and groupers were captured during the spring in the open Gulf of Mexico and during the fall along the continental shelf, most larvae were collected from the water column using bongo nets (Table 7.1). In addition, seabasses and groupers were distributed throughout the open Gulf as well as the continental shelf during spring and fall plankton surveys from 1982 through 2007 (Figure 7.46).

With few exceptions (1987 and 1988 during the spring and 1986, 1990, and 1993 during the fall), annual abundances for larval serranids averaged fewer than 6 per 10-min tow for spring and fall neuston net samples from 1982 through 2007 (Figures 7.47 and 7.48). Average serranid larval abundance for bongo net samples ranged from 9.4 (2007) to 49.4 (1994) per 10 m² and from 7.1 (1983) to 32.4 (1984) per 10 m² during spring and fall plankton surveys, respectively (Figures 7.47 and 7.48). For both spring and fall, the average abundance of seabasses and groupers was higher for bongo net samples than it was for neuston net samples.

Swordfish (Xiphias gladius) is the only species of the Xiphiidae family. This billfish is highly migratory and large, and while overfished, it has high value as a commercial and recreational species; as a top predator, swordfish play an important role in marine ecosystems (Rooker et al. 2012).

Swordfish larvae made up a very small percentage of the total ichthyoplankton catch; most were captured during the spring in the open Gulf at the water surface in neuston nets (Table 7.1). From 1982 through 2007, low numbers of swordfish larvae were collected by neuston net during the spring in the open Gulf, with average abundances ranging from 0 to 2.1 larvae per 10-min tow (Figure 7.49). Swordfish larvae were distributed sparsely throughout the open Gulf of Mexico during the spring. In the fall, they were occasionally found near the edge of the continental shelf (Figure 7.50). Swordfish larvae were collected by bongo net during spring plankton surveys in 1982, 1983, 1989, 1991, 1992, 1995, 1996, and 2004, with average annual abundances ranging from 0 to 7.6 larvae per 10 m². The average abundance of swordfish larvae collected by neuston net during the fall in 1986, 1988, 1989, 1995, 1998, and 2001 was 1 larva per 10-min tow, while the average abundance in 2000 was 2.3 larvae per 10-min tow. From 1982 through 2007, swordfish larvae were collected by bongo net only in 2001, with an average abundance of 4.9 larvae per 10 m².

The Gulf of Mexico continental shelf environment experiences seasonal changes in water temperature accompanied by discharges of low salinity, high nutrient water from rivers into the northern and eastern shelf areas. Using SEAMAP data, Muhling et al. (2012) characterized the spatial and temporal changes in abundances of larval fish assemblages on the northern Gulf of Mexico continental shelf from 1984 through 2008. Lanternfishes (Myctophidae) were the most common taxa collected and represented 14.65 % of the total collected ichthyoplankton, followed by codlets (Bregmacerotidae, 9.98 %) and gobies (Gobiidae, 9.29 %). Of the more than 500 taxa collected, the 20 most common fish families were evaluated. Larvae of some pelagic and mesopelagic families showed marked increases in abundance over the survey time period, while the abundances of some benthic fish families decreased (Muhling et al. 2012). Changes in fish assemblage structure were partially explained by changes in sea-surface temperature, as well as changes in the shrimp trawling effort. Interannual fish assemblage variability was also influenced by outflow from the Mississippi River. However, there was no explanation for spatial and temporal trends for many of the family groups (Muhling et al. 2012).

Carassou et al. (2012) investigated the spatial, seasonal, and depth-related structure of ichthyoplankton assemblages collected across a 77 km (47.8 mi) cross-shore gradient from March 2007 through December 2009 from highly productive estuarine waters to offshore oceanic waters on the Alabama shelf. A total of 350,766 larvae, in 17 orders and 70 families, were collected; the most common families were drums (Sciaenidae, approximately 42 % of total), followed by anchovies (Engraulidae, approximately 32 % of total). While the total density of fish larvae was significantly higher inshore, the number of families increased offshore. The total density of fish larvae also varied significantly among months, with the lowest values being observed in January and the highest in October and August. There were monthly variations in family richness, with minimum richness in December and maximum richness in May. Seven assemblages were associated with water masses characterized by distinct differences in temperature and salinity (Carassou et al. 2012). Families of larvae that were typically offshore included herrings, shads, sardines, and menhaden (Clupeidae); codlets (Bregmacerotidae); lizardfishes (Synodontidae); mackerels, tunas, and bonitos (Scombridae); and cusk-eels (Ophidiidae). Inshore families included anchovies (Engraulidae), gobies (Gobiidae), and clingfishes (Gobiesocidae). Larval fish assemblages varied seasonally and as a function of depth, but inshore and offshore assemblages remained clearly separated regardless of the season and depth considered; this strong and consistent structure was related to the combined effects of adult spawning behaviors and local oceanographic conditions, especially the influence of the Mobile River (Carassou et al. 2012).

Ichthyoplankton surveys were conducted in the northern Gulf of Mexico from 2006 through 2008 to determine the relative value of the region as early life habitat of sailfish (Istiophorus platypterus), blue marlin (Makaira nigricans), white marlin (Kajikia albida), and swordfish (Xiphias gladius) (Rooker et al. 2012). Sailfish were the dominant billfish collected in summer surveys, and larvae were present at 37.5 % of the stations sampled. Blue marlin and white marlin larvae were present at 25 % and 4.6 % of the stations sampled, respectively, and swordfish occurred at 17.2 % of the stations. Areas of peak production were detected and maximum density estimates for sailfish (22.09 larvae per 1,000 m²) were significantly higher than the other species: blue marlin (9.62 larvae per 1,000 m²), white marlin (5.44 larvae per 1000 m²), and swordfish (4.67 larvae per 1,000 m²) (Rooker et al. 2012). The distribution and abundance of billfish larvae varied spatially and temporally, and several environmental variables (sea-surface temperature, salinity, sea-surface height, distance to the LC, current velocity, water depth, and Sargassum biomass) were deemed to be influential variables. Densities of billfish were typically higher in frontal zones or areas proximal to the LC. Habitat suitability was strongly linked to physicochemical attributes of the water masses they inhabited, and observed abundance was higher in slope waters with lower sea-surface temperature and higher salinity. The study suggests that the northern Gulf of Mexico is very important in the early life habitat of billfishes (Rooker et al. 2012).

Tidwell et al. (2007) confirmed that the northern Gulf of Mexico provides important nursery habitat for billfish larvae. Ichthyoplankton surveys were conducted with neuston nets in the summers of 2005 and 2006 to identify areas in the northern Gulf of Mexico with high larval billfish densities. The mean density of larvae per sample ranged from 0 to 53.8 larvae per 1,000 m². The highest densities of billfish larvae were located at the fronts of anticyclonic eddies. The catch of 2,589 billfish larvae from 167 stations provides powerful support that the northern Gulf of Mexico is a billfish nursery.

Monthly samples of ichthyoplankton were collected from October 2004 through October 2006 from a site off the coast of Alabama in the northern Gulf of Mexico, about 18 km (11.2 mi) south of Dauphin Island, Alabama (Hernandez et al. 2010). Mean concentrations of total fish larvae peaked in August because of very high abundances of Atlantic bumper (290.6 larvae per 100 m³) and sand seatrout (Cynoscion arenarius, 301.1 larvae per 100 m³), while taxonomic diversity was generally higher from March through October. Taxonomic richness was generally highest during the late summer and early fall. Of the 58 different families of fish collected, the dominant groups included anchovies (Engraulidae), sand seatrout, Atlantic bumper, Atlantic croaker, Gulf menhaden, tonguefishes (Symphurus spp.), gobies (Gobiidae), drums (Sciaenidae), and cusk-eels (Ophidiidae) (Hernandez et al. 2010). Nearly all of the Atlantic bumpers (87 %) were collected in August, while sand seatrout were present throughout the year. The Atlantic croaker was the third most abundant taxon, with an October peak in abundance of 119.5 larvae per 100 m³ (Hernandez et al. 2010). It is important to note that the SEAMAP data are in units of number of larvae per 10 m² or per 10-min tow, whereas the Rooker et al. data are in numbers per 1,000 m² and the Hernandez et al. data are in numbers per 100 m³.

SEAMAP spring and fall surveys from 1982 through 2005 were analyzed to provide information on location and timing of spawning, larval distribution patterns, and interannual occurrence for groupers (Marancik et al. 2012). Shelf-edge habitat was determined to be important for spawning of many species of grouper. Spawning for some species may occur year round, but two peak seasons were evident: late winter and late summer through early fall. A shift in species dominance over the last three decades from spring-spawned species (most of the commercial species) to fall-spawned species also was documented.

The more than 4,000 oil and gas platforms in the Gulf of Mexico likely affect ichthyoplankton populations (Boswell et al. 2010). Lindquist et al. (2005) collected baseline information on vertical and horizontal distribution patterns of larval and juvenile fish from five offshore platforms off the Louisiana Coast from 1995 through 2000. Light traps and passively fished plankton nets were used at night to collect fish in surface and deep waters (15–23 m [49.2–75.4 ft] in depth) within the platform structure. Light traps were also used to collect fish from surface waters directly down-current of the platforms. Compared to light traps fished in deep water, light traps fished at the surface collected higher densities and diversities of ichthyoplankton. Herrings, shads, and sardines; anchovies; lizardfishes; and presettlement blennies were the most common in surface waters within the platforms, while postflexion mackerels and tunas and settlement-size blennies, damselfishes, and clownfishes were most common in surface waters down-current of the platforms. Deep plankton nets collected higher densities of non-herring/shad/sardine ichthyoplankton, while surface plankton nets collected higher numbers of taxa. The vertical distribution patterns described for dominant larval fish collected by plankton nets were generally consistent with those from other studies: herring/shad/sardine, jack, drum, and mackerel/tuna larvae more abundant in surface waters at platforms and lizardfish, codlet, goby, and left-eye flounder larvae more abundant in deeper waters (Lindquist et al. 2005).

Ditty et al. (2004) reviewed SEAMAP data from bongo net samples collected from 1982 through 1986 to describe the distribution of carangid larvae in the northern Gulf of Mexico relative to areas of high zooplankton. Of the 29,000 larvae from 13 species or species complexes in 11 genera, Atlantic bumper and round scad accounted for 91.7 % of all larvae, which agrees with the summaries above. Atlantic bumper densities averaged 2.9, 20.5, and 42.8 larvae per 100 m³ for the eastern, central, and western Gulf of Mexico, respectively, while densities of round scad averaged 6.7, 0.4, and 0.1 larvae per 100 m³, respectively, for the same regions. Carangids, including Atlantic bumper and round scad, appeared to spawn at water mass boundaries (fronts) and/or along other hydrographic features that promote higher productivity (Ditty et al. 2004).

The seasonal occurrence, distribution, and abundance of dolphinfish larvae were determined primarily from 814 neuston net collections taken during SEAMAP ichthyoplankton surveys of the Gulf of Mexico between 1982 and 1984 (Ditty et al. 2004). Larval dolphinfish were collected during all months sampled, but small larvae and pompano dolphin were found primarily during warm months. Larvae of common dolphinfish were significantly more abundant than pompano dolphin. Larval dolphinfish of both species were widely distributed in neritic and oceanic waters and most were collected near the surface. Over 90 % of common dolphinfish and about 80 % of pompano dolphin occurred over the outer continental shelf and in oceanic waters; overall densities averaged 4.8 and 0.8 larvae per 10 neuston tows, respectively.

The distribution, abundance, and seasonality of four carangids (blue runner, Atlantic bumpers, round scad, and rough scad, Trachurus lathami) off the Louisiana coast were evaluated using SEAMAP data from 1982 and 1983 (Shaw and Drullinger 1990b). Maximum abundances of larval blue runner, Atlantic bumper, and round scad were found in July inside the 40 m (131.2 ft) isobath. Larval Atlantic bumpers were captured in June and July only; blue runner in May, June, and July; and round scad in all seasons. Atlantic bumper larvae, concentrated mostly off western Louisiana, were by far the most abundant carangid in 1982 and 1983. Larval blue runner was the second most abundant summer-spawned carangid in 1982 and 1983; however, their abundance and depth distribution varied considerably between years (Shaw and Drullinger 1990b). The relative abundance of larval round scad off Louisiana was low, and they were captured only west of the Mississippi Delta. Rough scad were winter/spring and outer-shelf spawners; while they ranked third in overall abundance, they were the most abundant carangid on the outer shelf (Shaw and Drullinger 1990b).

Shaw and Drullinger (1990a) evaluated the distribution, abundance, and seasonality of four coastal pelagic species from the Clupeidae family—round herring, scaled sardine, Atlantic thread herring, and Spanish sardine—in the northern Gulf of Mexico using SEAMAP data from 1982 to 1983. During the summer, larval Atlantic thread herring and scaled and Spanish sardines were abundant on the inner shelf (less than 40 m or 131.2 ft) but were rare or absent in deeper waters. Scaled sardine and thread herring were found in all sampled inner-shelf water locations, but Spanish sardines were rare in the north-central Gulf (Shaw and Drullinger 1990a). During 1982, larval Atlantic thread herring were the most abundant of the four clupeids, while Spanish sardines were the most abundant during 1983. On the West Florida shelf, Spanish sardines dominated larval clupeid populations both years. Scaled sardine larvae were the least abundant of the four species both years; however, they were still captured in 20 % of the inner-shelf bongo net collections. Round herring larvae were collected from February through early June and were abundant on the outer shelf, especially off Louisiana. Over the 2-year period, outer-shelf mean abundance for round herring was 40.2 larvae per 10 m², while inner-shelf mean abundance for scaled sardine, Atlantic thread herring, and Spanish sardine were 14.9, 39.2, and 41.9 larvae per 10 m², respectively (Shaw and Drullinger 1990a).

Ichthyoplankton cruises were conducted in continental shelf waters off west Louisiana from December 1981 through April 1982 to determine the distribution and abundance of larval drums and croakers (Cowan and Shaw 1988). The total sciaenid larval density was highest in April, and the high densities were associated with the coastal boundary layer, a horizontal density front caused by an intrusion of freshwater from the Atchafalaya River east of the study area. Sand seatrout larvae were the most abundant, followed by Atlantic croaker, spot (Leiostomus xanthurus), black drum (Pogonias cromis), southern kingfish (Menticirrhus americanus), and banded drum (Larimus fasciatus). Spawning by sand seatrout began in January. Both sand seatrout and Atlantic croaker larvae were captured at higher rates at night than during the day (Cowan and Shaw 1988). Sand seatrout larvae appeared to be somewhat surface oriented, while spot may undergo a vertical migration.

Sogard et al. (1987) collected ichthyoplankton at three inshore–offshore transects off Southwest Pass, Louisiana, Cape Sand Blas, Florida, and Galveston, Texas, from 1979 through 1981 to determine densities of larval Gulf menhaden, Atlantic croaker, and spot in the northern Gulf of Mexico. All species were more abundant at inshore than offshore stations. Gulf menhaden and Atlantic croaker were most abundant off Southwest Pass, Louisiana, a major outlet of the Mississippi River. Of the three species, only the Gulf menhaden demonstrated any consistent vertical distribution pattern. At inshore stations Gulf menhaden were concentrated near the surface at midday, while offshore and present at 70 m (229.7 ft), most were also caught near the surface (Sogard et al. 1987).

Espinosa-Fuentes and Flores-Coto (2004) investigated the horizontal and vertical variation of ichthyoplankton assemblages in continental shelf waters of the southern Gulf of Mexico during each season in 1994 and 1995. A total of 21,814 ichthyoplankton, consisting of 25 families, 89 genera, and 92 species, was collected. Four assemblages were identified—coastal, inner neritic, outer neritic, and oceanic. Important members of the coastal assemblage in areas of the highest salinity fluctuations and in depths less than 30 m (98.4 ft) included estuarine-dependent species such as Atlantic bumper, sand weakfish, kingfishes (Menticirrhus spp.), croakers (Micropogonias spp.), and American stardrum (Stellifer lanceolatus). Abundant ichthyoplankton in the oceanic assemblage at depths of 50 and 100 m (164 and 328 ft) in areas with the least salinity fluctuations included pelagic species such as antenna cod (Bregmaceros atlanticus), lanternfishes (Myctophum spp.), pearly lanternfish (Myctophum nitidulum), large-finned lanternfish (Hygophum macrochir), and smallfin lanternfish (Benthosema suborbital) (Espinosa-Fuentes and Flores-Coto 2004). The main taxa in the inner neritic assemblage were hump-backed butterfish (Selene setapinnis), bigeye scad (Selar crumenophthalmus), shoal flounder (Syacium gunteri), eyed flounder (Bothus ocellatus), striped codlet (Bregmaceros cantori), and largehead hairtail (Trichiurus lepturus). The frequent and abundant species in the outer neritic assemblage of the outer-shelf stations and mid-depths were lanternfishes (Diaphus spp.), bristlemouths (Cyclothone spp.), fairy basslets (Anthias spp.), tunas (Thunnus spp.), bigeye scad, blue runner, rough scad (Trachurus lathami), bullet tuna (Auxis rochei), and striped codlet (Espinosa-Fuentes and Flores-Coto 2004).

Sanvicente-Añorve et al. (2000) evaluated the scales of the main physical and biological processes influencing the ichthyoplankton distribution in the southern Gulf of Mexico. These included the Bay of Campeche (spring 1983, winter 1984, and summer 1987), the littoral zone adjacent to Terminos Lagoon (bimonthly between July 1986 and May 1987), and the Carmen Inlet between the lagoon and the sea (monthly between April 1980 through January 1981). The main circulation patterns of the southern Gulf of Mexico, continental water discharges, mixing processes, and oceanic gyres were important processes affecting ichthyoplankton distribution patterns and community structure in the Bay of Campeche, and 81 families of ichthyoplankton, which included oceanic, neritic, and estuarine-dependent species, were collected (Sanvicente-Añorve et al. 2000). The neritic zone of the Bay of Campeche contained the highest densities of ichthyoplankton; highest densities (1,000–3,000 individuals per m³) were found near the Grijalva-Usumacinta River delta in the summer, and the lowest densities (fewer than 300 larvae per m³) were found in the winter. Distinct ichthyoplankton assemblages were identified and included a coastal assemblage characterized by Atlantic bumper (Chloroscombrus chrysurus, 5.4–209 larvae per m³), Atlantic thread herring (Opisthonema oglinum, 152 larvae per m³), sand weakfish (Cynoscion arenarius, 10.8–24 larvae per m³), Atlantic croaker (2.1–4.8 larvae per m³), and hogchoker (Trinectes maculatus, 3.9 larvae per m³); a neritic assemblage characterized by tonguefishes (Cynoglossidae, 2.1–6.7 larvae per m³), codlets (Bregmacerotidae, 5.8–63.6 larvae per m³), and left-eye flounders (Bothidae, 0.5–7.8 larvae per m³); and an oceanic assemblage dominated by lanternfishes (Myctophidae, 0.3–3.3 larvae per m³) and bristlemouths (Gonostomatidae, 0.1–2.3 larvae per m³). Twenty-three families of ichthyoplankton were collected from the littoral zone adjacent to Terminos Lagoon. Littoral currents, lagoon influence, spatial salinity variability, and meteorological conditions determined the structure and function of ichthyoplankton groups (Sanvicente-Añorve et al. 2000). In the littoral zone, a high abundance of ichthyoplankton occurred from May to September, followed by a strong decrease in January and March. While they changed in size, two groups occurred throughout the year; one group, which consisted of anchovies (46–197.6 larvae per m³) and gobies (8.5–371.5 larvae per m³), was typically located in the area adjacent to the Carmen Inlet. The second group, located near the Puerto Real inlet, was characterized by Atlantic thread herrings (49.4–56.4 larvae per m³), Atlantic bumpers (44 larvae per m³), and scaled sardines, 39.9 larvae per m³), which dominated in May, July, and September. In the Carmen Inlet between the lagoon and the sea, 38 families of ichthyoplankton were collected. Tidal- and wind-induced currents, bottom topography, and salinity gradients were the major forces controlling ichthyoplankton distribution (Sanvicente-Añorve et al. 2000). In the inlet, greatest densities of ichthyoplankton were found in the central-western section and the deepest eastern channel, and strong vertical stratification was observed; 99 % of the total catch consisted of anchovies, gobies, herrings/shads/sardines, drums, and mojarras. Distinctive ichthyoplankton patterns were produced by the combination of the physical, biological, and oceanographic processes and the life history strategies of the fishes—the periods and spawning areas of the adults, larval stages, dispersal capabilities of larvae, and the larval stage duration (Sanvicente-Añorve et al. 2000).