Title
A Mediterranean rocky-bottom ecosystem fisheries model
Author(s)
McClanahan T.R., Sala E.
Published
1997
Publisher
Ecological Modelling
Abstract
An aggregated energy-based simulation model of the Mediterranean infralittoral rocky-bottom was developed and used to carry out fishing experiments where fishing intensity and catch selection were varied. The model is composed of primary producers (algae), herbivores (sea urchins, vagile epifauna and herbivorous fish), suspensivores (sessile epifauna) and carnivores (piscivores and predators of invertebrates). Gross and net primary production are calculated from algal production and respiration parameters. Simulation results suggest that fishing affects the ecosystem's ecology and the benefits of fishing yields must be weighed against changes in the benthic ecology. Many of the changes are indirect effects caused by changes in trophic composition. The model predicts that yields of bony fish can vary by nearly a factor of 250 depending on the types of harvesting restrictions. The model predicts that fish depletion will result in a dominance of sea urchins, which will dramatically reduce algae, epifauna, and gross and net production. When harvesting of sea urchins is added to fisheries, the epifauna reaches high abundance levels once all herbivores have been removed. Maximum equilibrium yields for sea urchins can be quite high (~ 850 kg/ha per yr) but sea urchins are easily overfished and show large drops in yields and CPUE for small increases in effort. Because invertivorous fish (i.e. the sparid Diplodus) are the most important part of the finfish fishery, harvesting of sea urchins reduces fish yields and the effort at which maximum yields of fish are achieved by over 70%. Consequently, the value of sea urchins compared with fish is important in determining harvesting policies. Multiple guilds of herbivores keep algal abundance fairly stable over the full range of fishing intensity. Fishing only piscivores will result in a more evenly structured ecosystem, with dominance of herbivorous fish and, in this case, invertivorous fish, control sea urchin populations. Further, algal biomass, gross and net production are high, but yields of fish are minimal. Fishing only invertivores will also result in a dominance of sea urchins, and in a reduction of other fish by competitive exclusion. In this case, maximum fish yields are similar to values when fishing all fish resources, but fewer fishers are supported by the fishery.
Keywords
echinoderm fishery; enchinoderm fishery; finfish; finfish fishery; fishery model; fishery production; harvesting; modelling; modelling approach; sea urchin; yiueld; Meditteranean; Diplodus; Echinoidea
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