We examine alternative hypotheses for the decllne of 20 cod Gadus morhua stocks in the North Atlantlc The year of the lowest observed biomass of spawners did not correspond to low juvenile survival for the cohorts that should have contnbuted to the stock in that year However, fishing mortdl~ty was very high for the years preceding the collapse The collapse of the cod stocks was not caused by a lack of resilience at low population abundance because all spawners were able to produce many potential replacements at low population size We show that as populations collapsed, flshlng mortality increased untll the populations were reduced to very low levels We conclude that increased fishing mortality caused the population decl~nes, and often the collapses, of the cod stocks

We examined interannual variability in the timing of spawning of female cod Gadus rnorhua from 1947 to 1992 in 3 regions off Newfoundland, Canada, in the northwest Atlantic. Maturity data, assessed by visual examination of dissected gonads of cod collected by research trawls, were analysed with probit regressions to identify the day of each year on which 50% of females had ceased spawnlng (which we refer to as spawning time) on northern [Northwest Atlantic Fisheries Organisation (NAFO) Division 3L] and southern (3NO) Grand Bank and on St. Pierre Bank (3Ps). We minimized the bias in our estimates by separating sampling variability and interannual variation in age structure from true interannual variability in reproduction. Among regions, average spawning time (mean -t SD) varied from Days 157 + 18 (June 6 ) and 139 * 16 (May 19) on northern and southern Grand Bank, respectively, to Day 135 2 24 (May 15) on St. Pierre Bank. Interannual differences in spawning time were significant within 3L (1948-1991), 3 N 0 (1947-1992), and 3Ps (likelihood ratlo tests; p < 0.001). Interannual variation in spawning time was significantly associated wlth variation in water temperature prior to spawning in 3L and in 3Ps although the signs of the associations differed between regions, casting doubt on the hypothesis that the timing of cod reproduction represents an adaptive response to temperature change. The negative correlation between temperature and spawning time in 3L can be explained by the positive influence of temperature on gonad development. In 3Ps, we attribute the early spawning dates in years characterized by cold bank temperatures to (1) a thermal barrier Imposed by sub-zero temperatures on spawning migrations from the continental slope to the shelf, and to (2) increased rates of gonad development, and an earlier readiness to spawn, experienced by cod 'forced' to prolong their residence in warm slope waters. Our analyses indicate that cod spawning time varies significantly among years, demonstrate how the effects of temperature on cod reproduction depend on regional hydrography, and underscore the importance of separating variation in sampling protocol and age structure from true interannual variability in spawning time.

A new method is introduced for estimating selectivity of fishing gear from tagging data in which data from many experiments are combined. Selectivity is modeled as a multiplicative function of length and experiment effects using a generalized linear model with a log link function and a binomial error structure. We apply this method to 137 tagging experiments on Atlantic cod (Gadus morhua) conducted from 1954 to 1991. We show that the selectivity of otter trawls changed from the 1960s to the 1980s; during the earlier period the maximum probability of capture occurred at 55 cm and declined for longer fish, whereas in recent years the maximum probability is at approximately 60 cm and remains constant for longer fish. We discuss how selectivity estimates can be used to improve stock assessments.β & – 23

Department qf Biology, Dakholasie Uaziver.~ify, Wal$ux, N . S . , B3H 4J4 MYERS, W. A., AND W. W. DOYLE. 1983. Predicting natural mortality rates and reproduction-mortality trade-offs froni life history data. Can. J. Fish. Ayuat. Sci. 40: $12 -620.A method for estimating natural mortality and evolutionary constraint on fish life histories is presented based on the assumption that observed life histories are evolutionarily stable. Inverse optimization techniques are used to determine the values sf natural mortality, reproduction-mortality trade-offs, and energy conversion efficiencies that would make observed life histories evolutiona;ily stable. The life history method yields natural mortality estimates comparable with thost based on population age-frequency data. Sensitivity analysis is used to determine the robustness of the predictions to errors in parameter estimation and density-dependent factors. MYERS, R. A., AND R. W. DOYLE. 1983. Predicting natural mortality rates and reproduction-mortality trade-offs from life history data. Can. J. Fish. Aquat. Sci. 40: 612 -620. Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Depository Services Program on 09/20/13 For personal use only.

We use inverse optimization techniques and data from Landry (1978) to predict natural mortality rates in a population of the marine, planktonic copepod Acartia clausii. Predicted mortality rates are those that make the observed seasonal pattern of life-history characteristics evolutionarily stable. Our predictions closely approximate the rates observed by Landry. The results imply (1) that the inverse relationship between adult body size and temperature in A. clausii (also widely observed in other copepods and poikilotherms) is consistent with the hypothesis that it is an evolutionary adaptation to a seasonal environment, and (2) that demographic parameters, such as natural mortality rates, can be usefully estimated from life-history theory.