Status of Stocks & Multispecies Assessment Program
138th Annual Meeting of the American Fisheries Society
Drs. Olav Ormseth and Paul Spencer, SSMA Program, attended the 138th annual meeting of the American Fisheries Society (AFS), held in Ottawa, Canada, 17-21 August. The meeting theme "Fisheries in Flux" reflected our growing awareness of the dynamic nature of fish populations and ecosystems as well as changes occurring in the discipline of fisheries science. The meeting started with several keynote addresses, including a slide show on an expedition that retraced the 1940 Steinbeck-Ricketts expedition to the Sea of Cortez. Many changes have occurred there over the last 68 years, including a shift in the size distributions of many species towards smaller individuals and the expansion of a mesopelagic hypoxic zone.
At the awards ceremony, Dr. Ole Mathisen was recognized posthumously for his contributions to fisheries science in Alaska. Presentations at the AFS meeting occur within separate symposia, and there were a wide variety of topics ranging from sturgeon conservation to tagging. Of special interest was a symposium titled "Evolving Fish, Changing Fisheries," which focused on evolutionary changes induced by fishing. The speakers provided evidence for potential long-term effects of fishing through the use of modeling, laboratory, and field studies. This field is itself evolving, and the 3 days of the symposium were heavily attended. Elsewhere, the presentations by Ormseth and Spencer focused on the issue of how female attributes (e.g., age and size) affect offspring viability, and the implications of such effects for fisheries management. Abstracts of their presentations are provided below.
- The influences of maternal age of spawning, recruitment variability, and life-history pattern upon harvest reference points and fishery management:
Marine fish stocks exhibit a wide variety of responses to oceanographic variability and harvesting, reflecting largely differences in reproductive biology and stock-recruitment relationships. For some stock such as Pacific rockfish (genus Sebastes) and Atlantic cod (Gadus morhua), there is evidence that larval viability may be affected by the age of the spawner, thus potentially complicating stock-recruitment relationships. These maternal effects can be viewed as redefining the units of reproductive output from spawning stock biomass (i.e., eggs) to "viable larvae" (i.e., larvae remaining after mortality dependent upon spawner age is applied).
For Alaska Pacific ocean perch (Sebastes alutus), this redefinition results in estimates of Fmsy that are similar but slightly reduced from those obtained using spawning stock biomass as reproductive output, and steeper reductions in sustainable yield with fishing rates higher than Fmsy. Simulated "cod-like" and "rockfish-like" populations were used to further explore how estimates of management reference points such as Fmsy and Fcrash (the F level where equilibrium yield is reduced to zero) may be affected by life-history pattern, recruitment variability, and exploitation. In these simulations, larval survival was dependent upon spawner age, and estimates of Fmsy and Fcrash were made using either total larvae (proportional to eggs and spawning stock biomass) or viable larvae.
Over a range of harvest rates and levels of recruitment autocorrelation for each life-history type, estimates of Fmsy obtained when using total larvae were similar to those obtained when using viable larvae. However, estimates of Fcrash obtained when using total larvae were larger than those obtained when using viable larvae because of the pronounced truncation of age structure and decreased larval survival rates associated with the high fishing mortalities of Fcrash. This suggests that estimation of stock productivity at low stock sizes may be biased high if larval survival is dependent upon spawner age and this relationship is not reflected in the units of reproductive output utilized for stock-recruitment analyses and estimation of fishing rate reference points.
Reproductive potential of Pacific cod in Alaska:
In some species of marine fish such as Atlantic cod, older and larger females produce eggs and larvae with greater viability. Because the reproductive potential of these females is higher, the age structure of the population may be an important determinant of reproductive and recruitment success. To determine whether similar phenomena occur in Pacific cod (Gadus macrocephalus), I analyzed fecundity and egg size over a 4-year period in three regions of Alaska. Contrary to expectations, no maternal effects on egg size were observed. Females defend length-specific fecundity levels at the expense of egg size, implying that the number of eggs produced is the primary determinant of reproductive potential in Pacific cod.
It is possible that the maternal effects observed in species such as Atlantic cod result from fishery-induced early maturation of young fish. Therefore, the lack of such effects in Pacific cod may reflect the fact that similar changes in age structure and maturation have not occurred in Alaskan cod populations. Alternatively, there may be environmental factors that favor a reproductive strategy that relies on maximizing the number of eggs produced rather than the size of individual eggs.
By Olav Ormseth and Paul Spencer
U.S.-Norwegian Catch Estimation Workshop at the AFSC
On 16–19 September the AFSC hosted the second Norway-U.S. workshop on commercial catch sampling and estimation. This workshop was a follow-up on one hosted by Norway in February 2007. The theme of these workshops was to compare and contrast catch estimation and fishery sampling programs between Alaska, New England, and Norway. Participants from the United States included the NMFS Alaska Regional Office and AFSC staff to represent Alaska, and a number of scientists from the Northeast Fisheries Science Center and the University of Massachusetts Dartmouth to represent catch-sampling off of New England. The participants from Norway were from the Institute of Marine Research.
The workshop was organized into sessions discussing a) fishery data collection programs; b) the use of "study fleets" to estimate catch; and c) comparison of catch-at-age estimation methods and the implications of these sources of variability in stock assessments. Workshop participants drafted a set of documents based on these session topics, which when complete, will provide a valuable reference document and contribute to the literature on catch estimation methods. For example, a metric which provides an evaluation of catch-at-age precision was developed and will assist in providing specifications of estimation uncertainty in stock assessments.
