Effects of the Shrimp Fishery in the SE Gulf of California: Diminishing the Links of Aquatic Food Chains

Shrimp-trawl fisheries globally are estimated to produce one third of the total fishery discards. In the Gulf of California this bycatch is dominated by fish that have a low survival rate after capture, even if discarded, and this includes juvenile fish of commercial and non-commercial species. Although some fish are sold commercially the amount of fish discarded is still high (95% approximately).  This has contributed to the widespread belief that shrimp trawling causes numerous and detrimental changes to ecosystems. However, the effect of shrimp trawling on fish communities is generally unknown; attempts to examine the effect of shrimp trawling on the whole suite of bycatch fish species in the Gulf of California are scarce. Through the use of carbon (δ13C) and nitrogen (d15N) stable isotopes as well as stomach contents we analyzed an estuarine fish food web in the SE Gulf of California to determine the structure and function of the food web.  In this system a large small scale fleet operates to fish for shrimp. The 68 most abundant fish species were investigated, which accounted for more than 70% of the total fish biomass and abundance. The δ13C ranged from -19.27‰ (Fistularia corneta) to -13.25‰ (Diapterus peruvianus). The mean d15N values ranged from 11.36‰ (Chaetodipterus zonatus) to 18.90‰ (Arius platypogon). The food web is structured into five trophic levels. Most of the fish species analyzed are carnivores from trophic level 4 (45%-50%), followed by carnivores from level 3 (11%-17%) and 5 (9%-12%). The omnivores-detritivores (11%-15%) and planktivores (6%-9%) are primarily found in level 3. The herbivores (<5%) are at level 2. Most of the fish species found in the system belong to level 4, while there is a smaller proportion of species from the other levels. Predators from level 5 are scarce and species from level 4 do not serve as prey, and therefore, the abundance of level 4 is increased along with preying on species from levels 3 and 2 and reductions in abundance of species from these levels.  This could exert effects on the system, such as fishing down the food web (FDFW). Although this study was inconclusive, the evidence suggests that a FDFW phenomenon is occurring in this system, which results from the shrimp fishery removing the top trophic level from the system.