Science Advisory Report 2017/030
The Selection and Role of Limit Reference Points for Pacific Herring (Clupea pallasii) in British Columbia
- Limit reference points (LRPs) are defined as thresholds to states of serious harm to a fish stock where there may also be resultant impacts to the ecosystem, associated species and a long-term loss of fishing opportunities. Serious harm is considered to include irreversible and slowly reversible undesirable states.
- A LRP should be positioned before a state of serious harm occurs, rather than at the state of serious harm and must be avoided with high probability under the DFO Harvest Decision-making Framework Incorporating the Precautionary Approach (DFO 2009, hereafter called the DFO PA Framework).
- An evidence-based, production analysis approach, conditional on current data and stock assessment model assumptions, was used to evaluate whether the major Pacific Herring stocks in British Columbia show stock states consistent with signs of possible serious harm.
- Relationships between production and spawning biomass were examined to determine whether persistent low production and low biomass (LP-LB) states have occurred for the major stocks of Pacific Herring.
- The production analysis diagnosed recent LP-LB states for stocks in the Central Coast (CC), Haida Gwaii (HG) and West Coast of Vancouver Island (WCVI) management areas that were associated with persistent loss of benefits to resource users over a period from about one to two Pacific Herring generations (~6-11 years). Persistent LP-LB states were not diagnosed for stocks in the Prince Rupert District (PRD) and Strait of Georgia (SOG) management areas.
- The spawning biomass estimate that defined the upper boundary (frontier) of a persistent LP-LB state was interpreted as the threshold to possible serious harm for each Pacific Herring stock. A stock status-based LRP corresponding to the ratio of estimated spawning biomass (Bt) at the threshold to estimated unfished biomass (B0) was selected, based on results for the CC, HG, and WCVI stocks, across two assessment model configurations.
- A spawning biomass-based LRP of 0.3B0 is recommended for the CC, HG, and WCVI stocks based on results of the production analysis and consistency with international best practice recommendations.
- A LRP of 0.3B0 is also recommended for the PRD and SOG stocks as it aligns with best practice recommendations, and because these stocks are geographically adjacent to stocks for which recent low LP-LB states were detected.
- The equilibrium replacement fishing mortality rate, Frep, is considered a threshold for recruitment overfishing consistent with the concept of serious harm since it is a measure of the ability of a stock to replace itself over the long-term. Because of this, Frep and spawning potential ratio proxies, FSPR30 and FSPR40, were evaluated for each major stock, as were the equilibrium fishing mortalities associated with maximum sustainable yield, FMSY and F0.1.
- Estimates of Frep and proxies were implausibly high for the major stocks of Pacific Herring, largely due to increasing estimates of natural mortality (M) over time (non-stationarity). In addition, the juxtaposition of selectivity and maturity schedules suggested that all fish could spawn at least once before becoming vulnerable to commercial fisheries. Results also implied that stocks need to be maintained close to the unfished biomass, B0, to maintain population viability. Equilibrium reference points were rejected as candidates for LRPs due to numerous structural uncertainties that made their interpretation difficult.
- Experience with the current harvest strategy since 1986 indicates that persistent LP-LB states can occur when target harvest rates are set at or below 0.2 of the forecasted spawning biomass; for example, in the case of the CC, HG and WCVI stocks.
- A management strategy evaluation process, with engagement of managers and resource-users, is recommended to identify measurable objectives associated with both LRPs and target reference points for BC Pacific Herring. This process is needed for implementation of a closed loop simulation analysis to determine the expected performance of alternative management procedures with respect to providing acceptable performance and trade-offs in management outcomes related to the objectives.
- Ecosystem service requirements of Pacific Herring predators are poorly understood and measurable objectives for predators in BC are not specified. In the absence of quantitative models that represent hypotheses related to trophically dependent species, no adjustment of LRP recommendations for forage fish can be recommended at this time. Future development of operating models within a management strategy evaluation process may include ecosystem dynamics related to predator communities.
- Mechanisms to characterize serious harm to Pacific Herring stock in terms of states related to spatial distribution, stock structure, and genetic diversity are not well understood. Future development of population dynamics models that include spatial dynamics and/or stock structure may lead to candidate LRPs and performance indicators that characterize other definitions of serious harm. Spatial operating models could also inform management options at finer spatial scales than the current major management areas.
- It is recommended that the development of both operating and assessment models should focus on the parameterization of natural mortality, estimates of maturity-at-age, and the effects of assumed prior probability distributions.
- The phasing-in of any new management procedure (i.e., changes to data collection, stock assessment models and/or harvest control rules) designed to avoid LRPs and achieve targets is recommended to mitigate short-term consequences to resource users.
This Science Advisory Report is from the February 7-8, 2017 Regional Peer Review of The Selection and Role of Limit Reference Points for Pacific Herring (Clupea pallasii) in British Columbia, Canada. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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