Table 3. Input parameters for length-cohort analyses*. Term. F Term. F/Z Term. Z

Table 3. Input parameters for length-cohort analyses*.

Term. F Term. F/Z Term. Z Analysis

Standard 0.3 0.045 0.130 0.345

Cannibal 0.0 0.045 0.999 0.045

0.001 Cann. &

mammals

0.045 0.999

0.045 0.0

Cann., 2.85 mam. &

other

0.01- 1.10 0.82

0.045 0.045

Age-based from VPA

0.345 0.3-

1.0

0.130 0.045

In all analyses L~= 90 cm, number of length intervals = 17, length interval width

= 5 cm, total catch = 1.98X10' fish, K = 0.105. M is natural mortality, Term. F ia fishing mortality estimated for the terminal length class, from VPA. Term. Z is total mortality. Term. F/Z is the proportion of total mortality due to fishing, and M/K Is the

ratio of M to the von Bertalanffy growth parameter Jones 984!.

of the other analyses relative to my standard Fig. 1!. The LCA with M due only to cannibalism projected a higher population level, especially for fish in the smallest length categories, than the standard LCA, emphasizing the impact of cannibalism on population estimates. Most of the cannibalism removed pollock smaller than 15 cm. Although very high numbers of small fish would have been

produced to account for the estimated cannibalism, their contribution to total

biomass is still less than that from 40-cm and longer fish Fig. 2!.

Adding marine mammal predation to the LCA such that M consisted of cannibalism and mammal predation only! increased fish population estimates

Fig. 1! and increased mean biomasses among the middle pollock length groups Fig. 2!. The average size fish taken by marine mammals was slightly shorter

than the average size fish taken by the fishery. When subjected to marine mammal predation, the amount of biomass distributed among the longer pollock

length categories increased relative to both my standard LCA and the LCA with

cannibalism alone Fig. 2!.

Marine mammal predation and cannibalism are not the only natural mortality sources for pollock. Other sources of natural mortality were estimated in order

to gain an impression of just how high M might be. The additional M was

estimated by adjusting natural mortality in the model until total mortality Z! in the model conformed to that from the estimated long-term mean age composition

of Bering Sea walleye pollock. This final LCA estimated higher numbers of fish

in most length categories than in the previous analyses Fig. 1!. The resultant biomasses increased, especially in the middle length groups Fig. 2!. Converting the estimated numbers at length produced by this final LCA into biomass, and

estimating pollock egg production to complete the annual production cycle, I compared the relative impact of each estimated natural mortality source. Results

indicated that cannibalism removed the most biomass production from the

pollock population, marine mammal predation and other natural mortalities each

removed about 3/5 of the amount lost to cannibalism, and the commercial fishery removed the least of total pollock biomass production, about 1/4 of the amount lost to cannibalism. Computing portions of M from available data and then summing them to obtain total M resulted in a higher set of M values than using 0.3, the value used typically for total M in traditional cohort analyses.

Discussion

In most VPAs, M has been assumed to be constant, based on the notion that the relative changes in M for postrecruits are insignificant compared with the prerecruit changes in M. Recently, this assumption has been challenged see Vetter, 1988 for a review!. This study also challenges the use of constant M by illustrating the differences between population profiles for very young fish obtained when cannibalism is specified versus when cannibalism is ignored.

Currently, there are no direct estimates of relative numbers of pollock smaller than 10 cm age 0!. My study suggests that although age-0 fish appear to represent an insignificant part of the mean biomass, huge numbers may be produced each year Fig. 1!. Larger pollock are the target of both the

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Figure 2. Comparison of mean biomass distribution of walleye pollock as estimated from three different combinations of quantified natural mortality sources; cannibalism alone +!, cannibalism plus mammal predation *!, and finally cannibalism, mammal predation, and all other natural mortalities !.

commercial fishery and marine mammals. The level of mortalities due to fisKirtg and marine mammal predation thus affects the pollock population dirisctty- Additionally, through cannibalism, the levels of these mortalities have a urge secondary impact on the smaller size groups. This secondary impact needs tci be evaluated before fisheries scientists can accurately evaluate the effects cif changing levels of fishing mortality on the pollock population.

Marine mammal predation is similar to the fishery in that both operate as

apex predators on fish populations. They differ in that during the period

examined, marine mammals removed slightly smaller pollock than those reirioved

by the fishery. Swartzman and Haar 983 and references therein! also observed

that mammals eat smaller fish. If Alaskan sea lion populations are decreasing,

as recent research indicates Merrick et al., 1987!, then the average size of walleye pollock consumed by marine mammals may decrease to refieat the smaller sizes taken by the smaller northern fur seals. However, the mamrn4- fishery interaction may change as the exact length composition of pcifiock exploited by the fishery shifts according to the fleet vessel composition. In recerit years, the groundfish fleet composition has changed from foreign vessets to nearly exclusively domestic and joint venture JV! operations. The shift to domestic vessels has altered the areas fished, leading to potential changes I<

average size of fish landed. By separating components of M, this analysis ciffers an approach to separate the mortality pressures exerted by the different marine mammal groups and commercial fishing fleets and to estimate their relative

impacts on fish stocks.

Birds are also important fish-eaters, and presumably consume large numbers of Bering Sea walleye pollock. Furness 982! mentions several studies of

coastal areas where seabirds are estimated to consume over 26% cif fish biomass production. Frost and Lowry 984! estimated that Arctic ccid Boreogadus saida! made up as much as 90/o of the diets of the major se~4ird species present in the Alaska Beaufort Sea. Although seabirds in the eastern Bering Sea may not consume as high a percentage of fish producticiri as seabirds inhabiting other oceanic regions, including avian predation In tKis analysis would further increase estimated numbers of the smaller walleye pol ock

size groups.

The important contributions of this study are the potential for separating the different components of natural mortality to examine age or size-selactive mortality factors in a quantitative manner, and the estimates of retative magnitudes of mortality sources affecting pollock. These LCAs will improve as predation data sets improve. They would be fully testable only if methcida to survey directly all sizes of walleye pollock in the eastern Bering Sea became available. Even with the present resolvability of hydroacoustics data, it will be some time, if ever, before the numbers of age-0 pollock can be quantified. This extended LCA is a simple form of multispecies modelling providing tmth a method and an impetus to consider a species within its environment.

Bailey, K., and J. Dunn. 1979. Spring and summer foods of walleye pollock,

Theragra chalcogramma, in the eastern Bering Sea. Fish. Bull., U.S.

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gadids in northern Alaska and their body-otolith size relationships. Fish.

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