Graduate Thesis Or Dissertation
 

The effects of harvesting Pacific hake (Merluccius productus) on the offshore community : the results of a multi-species model

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  • The effects of harvesting Pacific hake (Merluccius productus) were examined with a multi-species numerical model composed of seven logistic growth equations coupled by a food web. The food web was composed of: California sea lions (Zalophus californianus), hake, sablefish (Anaplopoma fimbria), spiny dogfish (Squalus acanthias), a generic rockfish species (Sebastes spp.), Northern anchovy (Engraulis mordax), and a generic euphausiid species (as an example, Pacific krill, Euphausia pacifica). The model was tuned to mimic stock assessment biomass estimates for the years 1980 to 1991, after which a short term experiment and a sensitivity analysis were conducted. The short term experiment used a factorial design, with hake fishing and fishing for rockfish, sablefish, and anchovy as treatments. It was analyzed with scale analysis techniques. The results indicate that hake (from a management viewpoint) is potentially most important in the offshore community as a prey item for sea lions and as a predator on anchovies, but hake fishing mortality had a small effect on either population during the time span of the experiment. Results also suggest that hake has little or no competitive interaction with other species that are trophically similar. Increased harvesting of hake would probably do little to increase the biomass of rockfish or sablefish. The general conclusion of both the sensitivity analysis and the experiment is that species below their carrying capacity are mostly affected by changes in growth and removal processes while species close to their carrying capacity are mostly affected by processes controlling prey availability. A forty year projection from 1991 to 2031 was conducted to examine the effects of hake fishing on sea lion and anchovy biomass. Results indicate that sea lion biomass will vary inversely with hake fishing effort, while anchovy biomass is directly proportional to hake fishing effort. Results also indicate that hake experience environmental conditions not favorable to recruitment. During favorable conditions the hake population builds up a "surplus" that carries it through periods of unfavorable conditions. Increased hake fishing effort reduces the response of the population to favorable conditions. The results of this research constitute a step from the theory toward the practice of proactive multi-species and ecosystem management.
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