Comparative analysis of the community structure and trophic relations of the Peruvian hake...

Comparative analysis of the community structure and trophic relations of the Peruvian hake Merluccius gayi peruanus and its by-catch of the years 1985 and 2001 Michael Balln HCS 149ReferencesBianchi, G., Gislason, H., Graham, K., Hill, L., Jin, X., Koranteng, K., Manickchand-Heileman, S., Paya, I., Sainsbury, K., Sanchez, F., and Zwanenburg, K. 2000. "Impact of fishing on size composition and diversity of demersal fish communities." ICES Journal of Marine Science 57: 558-571.Daan, N., Gislason, H., Pope, J. G., and Rice, J. C. 2005."Changes in the North Sea fish community: evidence of indirect effects of fishing?" ICES Journal of Marine Science 62: 177-188.Pauly, D., Christensen, V., Dalsgaard, J., Froese, R., and Torres Jr., F. 1998. "Fishing down the marine food webs." Science 279: 860-863.Piet, G. J., and Jennings, S. 2005. "Response of potential fish community indicators to fishing." ICES Journal of Marine Science 62: 214-225.Wosnitza-Mendo, C., R. Guevara-Carrasco and M. Balln, 2004. Possible causes of the drastic decline in mean length of Peruvian hake in 1992. Bol. Inst. Mar Per 21 (1-2): 1-26. Zwanenburg, K. C. T. 2000. "The effects of fishing on demersal fish communities of the Scotian Shelf." ICES Journal of Marine Science 57: 503-509.IntroductionPeruvian hake preys on the most abundant species of the pelagic, demersal and mesopelagic systems. This species capitalizes on the abundances of its preys becoming the basis of the trawl fishery. However, the long term changes of the pelagic system (anchovy-sardine shift), the effect of El Nio and the intense fishing have depleted the Peruvian hake stock (Wosnitza-Mendo et al., 2004) to such an extend that the fishery was closed in September of 2002, the hake stock was declared overfished and new regulations were implemented to allow for its recovery.Nevertheless trawl fishing is a non-selective method and affects other species apart from hake. The continued removal of older and larger individuals from a demersal fish community, not only affects the abundance and size structure of the community but it also prompts the smaller size species to dominate the system (Daan et al., 2005; Piet and Jennings, 2005; Bianchi et al., 2000 and Zwanenburg, 2000). As large individuals generally feed in higher trophic levels (Pauly et al., 1998) these changes are usually linked to changes in the communitys trophic structure. The aim of this study is to infer changes in the north Peruvian demersal marine ecosystem by comparing the community structure and trophic relations of the Peruvian hake Merluccius gayi peruanus and by-catch between 1985 and 2001. The shapes of the size spectra of both years follow the hakes catch curves of the respective year (Fig.4). The community has changed in one direction, which is determined by the fishing effects, towards the small sized individuals.The drastic changes in hake size structure due to overfishing and the fact that the TLm of 2001 is determined mainly by juvenile hake (Fig. 5), which feed on euphausiids, indicate that the demersal fish community has experienced the fishing down the marine food web (Pauly et al., 1998) phenomenon. Fig. 5 Diet composition, trophic level and biomass distribution according to hake age groups.

It can be inferred from the comparison of the hake by-catch diets (Fig.6 and 7) of the two years and from its reduction in biomass that the macrobenthic community had a lower predation pressure in 2001 than in 1985, and that the observed increase in prey biomass in 2001 (Fig.8), was due to the biomass reduction of its main predators. Other factors could explain the differences in the benthic biomass between both periods too, such as the low and high oxygen concentration in 1985 and 2001, respectively. Nevertheless, if the observed increase in the benthic biomass resulted from its release from predation pressure, this would imply that the niche left by the benthic predators in 1985 was not filled by other species in 2001, suggesting that benthic production is top-down controlled. Results and Discussion

Although hake was distributed in a more restricted area and under condition of lower oxygen concentration (Fig. 2) the estimated biomass for 1985 was 400% higher than that for 2001(Fig. 1). Virtually all species present in 1985 had decreased drastically by 2001(Fig. 3). Species with larger sizes, long life and late maturity seem to be the most affected. This means that the community has experienced changes in species composition and reduction in biomass121086420246catfishsquidJumbo squidFig. 3 Biomass of the 12 most abundant species after hake.