Research Document 2017/014

Status of B.C. Pacific Herring (Clupea pallasii) in 2013 and forecasts for 2014

By Cleary, J.S., Taylor, N.G., and Haist, V.

Abstract

This document presents stock assessments for Pacific Herring (Clupea pallasii) in British Columbia waters using data current to 2013.  Results of the work are intended to serve as advice over the short term to fishery managers and stakeholders on current stock status and likely impacts of different harvest options. As in previous work, an integrated combined-sex statistical catch-at-age model (ISCAM) was applied independently to each of seven stock areas and tuned to fishery-independent spawn index data, annual estimates of commercial catch since 1951, and age composition data from the commercial fishery and from the test fishery charter program.  Assessments were done for five major stock areas: Haida Gwaii (HG), Prince Rupert District (PRD), Central Coast (CC), Strait of Georgia (SOG), and West Coast of Vancouver Island (WCVI), and two minor herring stock management areas: Haida Gwaii Area 2W, and WCVI Area 27. Results are summarized as stock reconstructions, status of spawning stock in 2013, and projected spawning biomass in 2014.

The model estimated stock-recruitment parameters, time-varying natural mortality, catchability coefficients for the survey time series, and selectivity parameters for the commercial fishery and those survey series for which age data are available.  Estimates of unfished equilibrium spawning biomass (SB₀) arose from Beverton-Holt stock-recruitment relationships (within the assessment model), calculated using average trends in weight-at-age, natural mortality and recruitment. All calculations were made using the Bayesian Markov Chain Monte Carlo (MCMC) method to quantify the uncertainty associated with parameter estimation.  Estimates of various quantities were calculated from these samples, and are presented as the 5 and 95% credible quantiles, with median values in parentheses.  Calculated probabilities are based on joint posterior distributions.  One-year spawning biomass projections for 2014 were performed for each major and minor stock area over a range of constant catches to estimate probabilities that spawning biomass and harvest rate metrics are below and above control points historically used in the management of Pacific Herring, as specified in the herring harvest control rule. 

The assessment shows current status of major stocks to have increased in 2013 over 2012 and the 2013 median posterior spawning biomass (SB₂₀₁₃) is estimated to be a greater proportion of estimated unfished equilibrium level (SB₀) compared to previous years. Ratio estimates (and 90% credibility intervals) of SB₂₀₁₃ / SB₀ by major stock area are as follows: HG = 0.80 (0.42-1.49); PRD = 0.49 (0.25-0.90); CC = 0.48 (0.29-0.78); SOG = 0.97 (0.62-1.48), and WCVI = 0.40 (0.23- 0.66). For HG, CC and WCVI stocks, where there have been no commercial removals in recent years, the model predicted increases in spawning biomass and recruitment, and decreases in natural mortality in order to explain observed increases in herring spawn index. Common observations across all five major herring stocks include: increases in the spawn index and spawning biomass, most significantly for HG, SOG and CC; declining weight-at-age from mid-1980s to 2010, and apparent decreases in the instantaneous natural mortality rate. For the minor areas, the assessment shows that Area 2W spawning stock biomass has been decreasing since 2011, driven by a declining spawn index since 2010 and apparent increases in the instantaneous natural mortality. For Area 27, the assessment shows that the spawning stock biomass has been at a constant level over the past 10-years but instantaneous natural mortality has been decreasing. Ratio estimates (and 90% credibility intervals) of SB₂₀₁₃ / SB₀ by minor stock area are: Area 2W =1.10 (0.45-2.33) and Area 27 = 0.55 (0.31-0.98).

Uncertainty associated with truncated catch data (1972 – present) and an adjustment in the spawn index to account for lead-line changes (2003-2013) was explored through sensitivity runs of the major areas. Fitting the model to a truncated time series had varying effects between stock areas whereas adjustments to spawn widths had negligible influence on model output.

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