Historical variations in pelagic fish communities and their causes.
Date
2007
Authors
Ward, Peter.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
I explore bias in body-size and abundance indices that are derived from pelagic longline data, then describe historical variations in pelagic fish communities, and review possible causes of those variations. Most species of large, predatory tuna, billfish, and sharks, were characterized by significant declines in abundance soon after longline fishing commenced, followed by long-term stability. The largest and most abundant predators, such as sharks and large tunas, suffered the greatest declines in abundance. They also showed striking reductions in mean body-size.
I show how longline fishers have modified their fishing gear and practices to improve fishing power and catchability, which has altered the relationship between catch rates and abundance. Variations in hook depth, bait species, the local abundance of tuna, and the timing of longline operations influenced the rate of bait loss from pelagic longlines. A reduction in soak time since longlining commenced in the 1950s has introduced a systematic bias in estimates of mortality levels and abundance of most species. Habitat models are shown to be inaccurate in predicting the depth distribution of bigeye tuna. A new method that uses generalized linear mixed models is developed to infer the depth distribution of pelagic fishes. My analyses do not reveal anything unique about longline gear or pelagic animals that might explain the community variations.
I review hypotheses that explain the observed variations in pelagic fish communities. Avenues of future research that may help to explain the variations include studies of the removal of an accumulated biomass and changes in schooling behaviour related to density thresholds.
Thesis (Ph.D.)--Dalhousie University (Canada), 2007.
I show how longline fishers have modified their fishing gear and practices to improve fishing power and catchability, which has altered the relationship between catch rates and abundance. Variations in hook depth, bait species, the local abundance of tuna, and the timing of longline operations influenced the rate of bait loss from pelagic longlines. A reduction in soak time since longlining commenced in the 1950s has introduced a systematic bias in estimates of mortality levels and abundance of most species. Habitat models are shown to be inaccurate in predicting the depth distribution of bigeye tuna. A new method that uses generalized linear mixed models is developed to infer the depth distribution of pelagic fishes. My analyses do not reveal anything unique about longline gear or pelagic animals that might explain the community variations.
I review hypotheses that explain the observed variations in pelagic fish communities. Avenues of future research that may help to explain the variations include studies of the removal of an accumulated biomass and changes in schooling behaviour related to density thresholds.
Thesis (Ph.D.)--Dalhousie University (Canada), 2007.
Keywords
Biology, Oceanography., Agriculture, Fisheries and Aquaculture.