Proceedings 2011/075
Proceedings of the Regional Advisory Meeting on the biological sampling of Pacific Herring, and factors influencing the variability in Pacific Herring egg layers and considerations to stock assessment; January 18-20, 2011
Chairperson: S. MacConnachie
Summary
Given reductions in resources for data collection following the Larocque court decision, concern has been expressed about the adequacy of ongoing data collection programs. Consequently, there is ongoing interest in exploring the cost-benefits and tradeoffs of varying spatial and temporal sampling coverage versus the precision of parameter estimates. Conclusions and recommendations from several past reviews have also identified this need. These types of evaluations are required in order recognize whether datasets in the time series can be used to distinguish different biological characteristics between regional stock groupings. Despite area closures in three of the five major regional assessment areas, the biological sampling and spawn survey programs operate annually. However, there is some concern about the adequacy of the biological sampling program in providing information on fish size and age composition of major herring stocks for stock assessment analysis and modelling.
Although herring spawn data have been collected for over 50 years, a detailed study of factors that influence the number of egg layers deposited has not been conducted. A better understanding of factors governing the density of spawn deposition will provide valuable information on herring reproduction. The assumption that low numbers of egg layers in a specific spawning site is symptomatic of a low spawning biomass should be investigated since this view may be implied by stock assessment sampling and modelling.
Mortality of eggs during spawning has been examined and documented at most major spawning sites, both in the Atlantic and Pacific Ocean. The loss of eggs during spawning has special relevance to Pacific herring stock assessments that rely on a quantitative index of herring spawn as a key component for annual assessments. A particular concern is that as the financial and logistical support for spawn surveys has diminished the timing of the surveys may be relatively later than during earlier surveys. Relatively later assessments of spawning by SCUBA surveys could result in an under-estimate of spawn, hence an underestimate of the spawning biomass. The potential scale of such possible under-estimates is uncertain, but even a relatively small daily loss (~2%) would result in a total loss of over 25% during a 14-day incubation period. A daily loss of 5% would result in total reduction of more than 50% during the same period. Also, there are other uncertainties that affect the estimates of spawn survival, including density dependent survival of eggs, with survival to hatching being lower in very high densities, etc. It is plausible that this is a concern for areas of the BC coast where spawn has tended to concentrate in fewer areas in recent years.
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