Dynamic Changes in Marine Ecosystems Fishing, Food Webs, and Future Options (2006) / Chapter Skim
Currently Skimming:
2 Evidence for Ecosystem Effects of Fishing
2 Evidence for Ecosystem Effects of Fishing
Pages 23-58
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From page 23... ...
Yet, fishing is both size selective and species selective, meaning that the abundance and mean size of fished species are often reduced, and the genetic structures of populations are potentially altered. Furthermore, species interactions are often complex and fishing can modify elaborate connections in marine communities and food webs.
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From page 24... ...
fishing down and through food webs; and (4) inducing regime shifts through either physical or biological forcing.
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From page 25... ...
In these open-ocean communities, catch rates fell from 612 individuals per 100 hooks down to 0.52 during the first 10 years of exploitation. Relative biomass estimates from the beginning of industrialized fishing (solid points)
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Based on all their observations, Cox et al. conclude that the changes generally represent decreases in top predators and subsequent increases in small tunas, on which the top predators prey.
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27 (tonnes) Catch (tonnes)
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From page 28... ...
Production likely increased because cannibalism by and competition with the larger individuals was reduced, and the fishery was then able to take a higher proportion of individuals from slightly lower trophic levels. Polacheck (in press)
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. At the very least, evidence reveals that the likelihood of fishery-induced regime shifts (discussed later in the chapter)
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From page 30... ...
, per 1000 hooks (species density, closed diamonds) , and total catch (squares)
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Often the bycatch species of the target fishery are those most in danger of precipitous declines since fishing effort is not influenced by their abundance. Bycatch increases the extinction probabilities for a growing number of non-target species.
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From page 32... ...
While there have been few global extinctions as a result of fishing, the potential exists that future extinctions -- including ecological extinctions -- are possible, contrary to past thinking. The data presented for the declines in biodiversity and new examinations into the extinction potential for marine fish indicate that these issues deserve consideration in fisheries management decisions, especially as managers try to incorporate larger ecosystem concerns (the multispecies nature of fisheries management, accounting for effects of predation and competition)
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From page 33... ...
(2004) show that fishing pressure did result in genetic changes for northern cod.
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From page 34... ...
Sufficient historical data to quantify the roles for the major consumers in these communities (e.g., sea otters, large fishes, urchins, abalones) before exploitation are absent.
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From page 35... ...
. ALTERED FOOD WEBS Fisheries not only affect populations but also alter the energy flows and species interactions in marine food webs and communities simply because all fished species are components of food webs and interact with other species through predation and competition.
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Food Webs and Trophic Interactions Food webs are the road maps to known or even imagined species interactions and therefore display ecological connections. Their use as a descriptive tool extends back to the 1800s.
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From page 37... ...
Ecosystems are biologically complex, and food webs seem the most appropriate vehicle to display the implied interactions and their potential consequences
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From page 38... ...
38 these of strength the but shown, are interactions Direct Ocean. Pacific central the for web food descriptive a of 1999.
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39 each of biomass line. relative connecting the example, of Administration.
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40 DYNAMIC CHANGES IN MARINE ECOSYSTEMS FIGURE 2.6 Interaction webs show the strength of species interactions, where the per capita interaction strength is indicated by the width of the arrow, and how these interactions change due to perturbation. For this example, the perturbation is the introduction of orcas into a food web previously dominated by otters.
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From page 41... ...
. More recently, the importance of trophic cascades or the selective influence of consumers on the abundance and productivity of food-web components at lower trophic levels has emerged and is now recognized as possibly of equal importance in determining the dynamics of marine food webs (Estes et al.
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From page 42... ...
. Analyses of the statistical relations between different trophic levels in marine ecosystems provide evidence for both strong consumer effects (top-down)
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From page 43... ...
. Certain species exert strong controlling influences on marine food webs and these special species have an unexpectedly large impact on food-web structure and function.
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From page 44... ...
pressures do occur and reverberate through trophic cascades in complex food webs and ecosystems. Sequences of events are set in motion and reveal themselves over decades.
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From page 45... ...
FISHING DOWN AND THROUGH THE FOOD WEB Long-term reductions in the mean trophic levels of fisheries landings have been measured worldwide. Explanations as to why trophic level changes in fisheries landings have occurred include (1)
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From page 46... ...
46 AAS. Pacific, A 1998 North © (A)
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From page 47... ...
The second, fishing through the food web, involves serial additions of lower-trophic-level species to the catches. Declines in mean trophic level by fishing through the food web can be caused by initial depletion of predators in the early stages of a fishery, followed by release of prey from predation, thereby increasing their abundance and their contribution to the fishery.
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From page 48... ...
exhibited a similar decline in mean trophic level over the same time period, but landings of high-trophic level species generally increased over this period, while new fisheries developed for lower trophic species. SOURCE: Essington et al.
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From page 49... ...
Trophic Efficiency of Food Webs Irrespective of what causes the change in the mean trophic levels of landings, a reduction in mean trophic level of an ecosystem would be expected to result in an increase in productivity available for fisheries harvest. Transfer efficiency
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From page 50... ...
In a contrasting example for global tuna and billfishes as a group, while mean trophic level declined from 1950 to 2000, the FIB index steadily increased, suggesting the possibility of some yield benefits owing to fishing at lower trophic levels. Pauly and Palomares (2005)
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From page 51... ...
reports that changes in biological baselines in response to regime shifts exhibit several characteristics. These are evident when abundances of various species covering a range of trophic levels that persist around long-term baselines suddenly and coherently shift to a new baseline level where they again persist.
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From page 52... ...
reports other examples of fish population responses to climate regime shifts in the North Pacific. Biological responses to regime shifts, especially in upper trophic levels, can lag or be masked by other processes and events (Miller and Schneider 2000)
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From page 53... ...
Once a stock has been depleted by fishing, complex competition and predator-prey interactions may prevent reversal after fishing has ceased. For example, one hypothesis proposed to explain the lack of recovery of Newfoundland cod involves increased predation on cod juveniles by a predator that, prior to the cod collapse, was kept under control by cod predation (Walters and Kitchell 2001)
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From page 54... ...
This fact alone could have a great deal of bearing on the way an ecosystem responds to the application or reduction of fishing pressure. In a relevant review, Rose et al.
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From page 55... ...
If recovery of populations and food webs to earlier states is determined to be desirable, it will usually be a slow process, especially for fishes with older ages of maturation. In some cases recovery may not occur.
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From page 56... ...
Reduced biomass and abundance, changes in size structure, genetic changes, and changes in trophic structure of ecosystems (including alternative states and altered productivity, species interactions, trophic cascades, and food webs) have all been documented as consequences of fishing.
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From page 57... ...
Realizing that there is a theoretical limit to the productivity that can be taken from the oceans and that we may currently be at or approaching that limit, food-web interactions will become increasingly important in future fisheries management decisions. With the inability to change one ecosystem component without affecting numerous others, society will need to determine which ecosystem components are the most desirable for harvest, and then managers will need to implement policies designed to maximize this desired production while recognizing that this will affect other species.
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