Changes in trophic flows and ecosystem properties of the Beibu Gulf ecosystem before and after the collapse of fish stocks

Abstract

Mass-balance models (Ecopath) of the ecosystem before and after collapse (1959–1961 and 1997–1999) of fish stocks were developed with Ecopath software to compare the differences in ecosystem structure, functioning and ecosystem properties of the Beibu Gulf. The model includes 20 functional groups consisting of commercial important fish groups and other ecologically important groups in the ecosystem such as zooplankton, phytoplankton, and detritus. Results indicated that biomass and catches of the system have changed drastically between the 1960s and 1990s, especially for the high trophic levels (TL). The biomass of level V in the early 1960s was 32 times higher than that of the late 1990s, however, the biomass of level I and II in the 1990s was higher than the 1960s. Despite the higher catches in the 1990s, fishing was ecologically less expensive during the 1990s than 1960s due to small fish catches were large. Mean transfer efficiency decreased from for 10.2% in the 1960s to 9.1% in the 1990s periods. According to the summary statistics, the parameters of net system production (NPS) and total primary production to total respiration ratio were increased from 1.013 in the 1960s to 2.184 in the 1990s, however, the connectance index (CI), system omnivore index, Finn’s cycling index and mean path length decreased from the 1960s to the 1990s. The overhead (O) was higher in the 1990s model while the ascendancy (A) decreased nearly 10% in the 1960s. The ‘Keystoneness’ result indicate that zooplankton was identified as keystone species in 1960s, however, the elasmobranches was keystone species in the late 1990s. The average trophic level of the fishery decreased from 3.32 in the 1960s to 2.98 in the 1990s, and exhibits classic symptoms of “fishing down the food web”. All the indices of the system attributes suggests that the Beibu Gulf ecosystem in 1960s was found to be more mature than in the 1990s due to the collapse of demersal ecosystem, and the ecosystem changed from being dominated by long-lived, high trophic level groundfish dominated system toward a system with small-size and low-value species over fifty years.

Introduction

Fishing activities have altered and degraded marine ecosystems through both direct and indirect effects, especially in coastal regions where fishing and other anthropogenic perturbations are most intense (Botsford et al., 1997, Pauly et al., 2002, Dulvy et al., 2004, Hilborn et al., 2004). Over the past few decades, fishery-induced collapses of 90% large predatory fishes have occurred throughout the world (Myers and Worm, 2003). The Beibu Gulf ecosystem is no exception to this common picture.

The Beibu Gulf ecosystem is very productive and its living marine resources have been exploited for nearly half a century. Historically, the system has been very important as a fishing ground in the South China Sea. However, with the engine boats introduced in the Gulf since the 1960s, the fishing effort and catches increased drastically during the past four decades (Chen and Qiu, 2002). With heavy exploitation, the catch compositions had changed significantly. Red snapper Lutjanus sanguineus was a large demersal fish and a dominant commercial species in the Gulf. The annual landing of the snapper in the Beibu Gulf markedly declined from137.2 kg km⁻² in 1960s to 2.5 kg km⁻² in 1992 (Yuan, 1995).Consequently, the catches became dominated by substantial amounts of juveniles and small pelagic species, and small-size, low-value fishes prevailed instead of high quality, high trophic level, large-size fishes. Large demersal fishes including groupers (Serranidae), snappers (Lutjanidae), yellow croakers and giant croakers (Sciaendiae) etc., which were traditionally targeted by the trawl fisheries, are now depleted (Sun and Lin, 2004, Qiu et al., 2010).

The Beibu Gulf has been relatively well studied during the last 20 years. Over this time period, large amounts of data have been collected and analyzed and sampling techniques have also become more sophisticated. However, previous studies in this area have consisted of fisheries surveys (Zeng et al., 1989, Luo et al., 1999) or focused on individual species (Cai et al., 1986, Qiu, 1996), there is few studies have been constructed on species interactions or on the ecosystem effects of fishing in the Gulf (Cheung and Sumaila, 2008). The ecological modeling software, Ecopath is a “snapshot” of given ecosystem and practical method to quantitatively improve knowledge on structure and functioning of different ecosystems. Among the analyze of exploited ecosystems undertaken using the EwE approach, the comparison of ecological models representing different situation of a given ecosystem through time has been shown to be a useful exercise (Shannon et al., 2003, Heymans et al., 2004).

In this paper, two periods are modeled: the early 1960s, before the collapse of large groundfish, and the late 1990s, after the collapse. The results were used to evaluate and compare the structure and function between the two periods. The changes in the trophic interactions, the community structure and function of the ecosystem during the half century were analyzed and evaluated to gain an insight into the status of the ecosystem development. This work also serves as a basis for future comparisons of the results of possible consequences of fisheries management scenarios derived from static networks of trophic flows to dynamic simulations of fishing impacts.