Jon Brodziak

Implications of recent increases in catches on the dynamics of Northwest Atlantic spiny dogfish (Squalus acanthias)

US commercial landings of spiny dogfish have increased five-fold since 1987 to over 22 000 mt in 1993. Over 95% of the landings consist of mature females80 cm in length. Minimum discard estimates for 1993 suggest an additional 25 000 mt of dogfish were discarded, of which 14 000 mt were killed. LOWESS-smoothed minimum swept-area biomass estimates reveal a five-fold increase in abundance over the last 30 years to about 650 000 mt in 1993. There is no evidence, however, of a continuing increase in the fishable stock (i.e.,80 cm in length) since 1990. Mean lengths of dogfish in commercial landings and research survey catches have decreased in the last five years, and fraction of females >80 cm in the survey has declined markedly. Change-in-sex-ratio estimators were used to derive sex-specific F values. Results suggest a five-fold increase in F on females from 1991 to 1993 to levels in excess of 0.25 per year. Commercial fishery information, research survey data, and life history parameters from the literature were used to develop a size- and sex-structured equilibrium model for dogfish. Using the concept of pups per recruit, preliminary biological reference points for fishing mortality were derived. With a minimum size limit of about 80 cm, fishing mortality rates (F) in excess of 0.2 would lead to the gradual decline of the spiny dogfish stock. While current minimum biomass estimates are high, mature females may already be overexploited. In view of the delayed maturation, low birth rates, and longevity of this species and experiences in shark fisheries worldwide, plans to increase exploitation rates should proceed cautiously to avoid rapid depletion of the adult stock. # 1998 Elsevier Science B.V. All rights reserved.

Effects of Stochasticity in Early Life History on Steepness and Population Growth Rate Estimates: An Illustration on Atlantic Bluefin Tuna

The intrinsic population growth rate (r) of the surplus production function used in the biomass dynamic model and the steepness (h) of the stock-recruitment relationship used in age-structured population dynamics models are two key parameters in fish stock assessment. There is generally insufficient information in the data to estimate these parameters that thus have to be constrained. We developed methods to directly estimate the probability distributions of r and h for the Atlantic bluefin tuna (Thunnus thynnus, Scombridae), using all available biological and ecological information. We examined the existing literature to define appropriate probability distributions of key life history parameters associated with intrinsic growth rate and steepness, paying particular attention to the natural mortality for early life history stages. The estimated probability distribution of the population intrinsic growth rate was weakly informative, with an estimated mean ru200a=u200a0.77 (±0.53) and an interquartile range of (0.34, 1.12). The estimated distribution of h was more informative, but also strongly asymmetric with an estimated mean hu200a=u200a0.89 (±0.20) and a median of 0.99. We note that these two key demographic parameters strongly depend on the distribution of early life history mortality rate (M0), which is known to exhibit high year-to-year variations. This variability results in a widely spread distribution of M0 that affects the distribution of the intrinsic population growth rate and further makes the spawning stock biomass an inadequate proxy to predict recruitment levels.

Development of Bayesian production models for assessing the North Pacific swordfish population

Bayesian surplus production models were developed for assessing the North Pacific swordfish population under alternative scenarios: a single-stock scenario and a two-stock scenario with subareas that represented the western central and eastern Pacific Ocean regions. Biomass production was modeled with a three-parameter production model that allowed production to vary from the symmetric Schaefer curve using an estimated shape parameter. Lognormal prior distributions for intrinsic growth rate and carrying capacity were assumed. Goodness-of-fit diagnostics were developed for comparing the fits of alternative model configurations based on the root-mean squared error of catch per unit effort (CPUE) fits and the standardized CPUE residuals. Production model fits for 1952–2006 indicated that the Japanese longline CPUE numbers were influential under each stock scenario because these scenarios were the longest time series of relative abundance indices. Model results also indicated that assumptions about the prior means for intrinsic growth rate and carrying capacity may be important based on the model configuration.

Stock assessment update for the main Hawaiian Islands Deep 7 bottomfish complex through 2013 with projected annual catch limits through 2016

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Maximum Likelihood Estimation of Stock Composition

Publisher Summary Stock composition can be a significant uncertainty in the management of mixed-stock fisheries of highly migratory or transboundary resources. When a mixture of stocks with different productivities is harvested together, stocks with low productivity may be overharvested. Conversely, forgone yield may be substantial for high-productivity stocks if mixed-stock fishing effort is set too low. In this context, mixed-stock fisheries are similar to multispecies fisheries where different life histories imply different target harvest rates across species. Last, allocation of harvest among user groups is often predicated upon knowing the stock composition of the catch. Measuring stock composition in mixed-stock fisheries is often more challenging than determining species composition in multispecies fisheries, since identifying species is generally much simpler than determining where fish come from. One of the key challenges to effective management of mixed-stock fisheries is estimation of the relative contribution of each stock to the harvest. Stock composition analysis uses either mark-recapture techniques or differences in the frequency distributions of population characteristics among stocks to estimate the composition of a mixture of stocks. For example, statistical analyses of allozyme frequency distributions have routinely been used to estimate the contribution of Pacific salmon stocks in mixed-stock ocean fisheries. Seasonal fluctuations in the spatial distribution of stocks along with changes in the magnitude and distribution of fishing effort can further complicate stock composition analysis.

Estimating Total Mortality Rates from Mean Lengths and Catch Rates in Nonequilibrium Situations

AbstractA series of estimates of the total mortality rate (Z) can be obtained by using the Beverton–Holt nonequilibrium-based approach of Gedamke and Hoenig (2006) on observations of population mean length over time (ML model). In contrast, only relative mortality rates (not absolute values) can be obtained from a time series of catch rates. We derived the transitional behavior of the catch rate following a change in total mortality in the population. From this derivation, we developed a new method to estimate Z that utilizes both mean lengths and catch rates (MLCR model). Both the ML model and the MLCR model assume constant recruitment in the population. We used a simulation study to test performance when recruitment is variable. Simulations over various scenarios of Z and recruitment variability showed that there may be correlated residuals in the mean lengths and catch rates arising from fluctuations in recruitment. However, the root mean square errors of the Z estimates and the change point (i.e., the...

Stock assessment updates of the bottomfish management unit species of American Samoa, the Commonwealth of the Northern Mariana Islands, and Guam in 2015 using data through 2013

In this report, we conduct a strict stock assessment update of the Bottomfish Management Unit Species (BMUS) in Guam, American Samoa, and the Commonwealth of the Northern Mariana Islands using the same base case production model as used in the previous stock assessment (Brodziak et al. 2012), but with an additional 3 years of catch and nominal CPUE as input data. A Bayesian statistical framework is applied to estimate parameters of a production model fit to a time series of annual CPUE statistics to provide direct estimates of parameter uncertainty for status determination. The surplus production model includes both process error in biomass production dynamics and observation error in the catch-per-unit effort data. Overall, the American Samoa, the Commonwealth of the Northern Mariana Islands, and the Guam bottomfish complexes were not overfished (overfished is defined as B<0.7*BMSY) and were not experiencing overfishing (overfishing is defined as H>HMSY) in 2013, the most recent year of the stock assessment estimates. We conducted stock projections for 2016 and 2017, which projected a range of hypothetical twoyear catches and calculated corresponding risks of overfishing. For the American Samoa BMUS complex, the 2016 catch level that would produce a 50% risk of overfishing in 2016 was 137 thousand pounds. For the Commonwealth of the Northern Mariana Islands BMUS complex, the 2016 catch level that would produce a 50% risk of overfishing in 2016 was 304 thousand pounds. For the Guam BMUS complex, the 2016 catch level that would produce a 50% risk of overfishing in 2016 was 82 thousand pounds. All of these catch values associated with a 50% risk of overfishing in 2016 are much higher than actual bottomfish landings in 2013 for American Samoa, the Commonwealth of the Northern Mariana Islands, and Guam which were 23,630, 22,510, and 29,848 pounds, respectively.