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The methods used to assess the Barents Sea capelin, Mallotus villosus (Mü ller), stock have always differed considerably from those used for most other stocks. Owing to the life history characteristics of capelin (short lifespan, considerable natural mortality, including spawning mortality), sequential population analysis (VPA, etc) cannot be applied to assess stock status and to generate catch prognoses. The assessment is based on an annual acoustic survey, which is regarded as an absolute measurement of stock size. The assessment methods have changed considerably since these activities started in the early 1970s. They have evolved from a relatively simple single-species model, used prior to the first major stock collapse in the early 1980s, through several steps towards the rather ambitious probabilistic model used at present, in which multispecies interactions and uncertainty in measured and modelled quantities are included. In this paper we review the history of stock assessment of Barents Sea capelin, describe current practice, and briefly outline the extensions to the model that will soon be implemented. Current management practice is evaluated in light of the history of the stock and the fishery, and is compared with that used with other capelin stocks.

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Since 1998, the assessment model framework Bifrost/Captool has been used for advice on total allowable catch for the Barents Sea capelin (Mallotus villosus) stock. However, since the management is based on a target escapement strategy, and most capelin die after spawning, viewed in retrospect, there is hardly any possibility of checking whether the forecast underpinning the quota was actually realistic. The forecast using Captool for the period from 1 October up to capelin spawning time at 1 April relies on a forecast of cod abundance during this period. This estimate of cod abundance is, in turn, based on a cod assessment from the previous spring. By rerunning the Captool model, where the cod forecast is replaced with the actual amount of cod from the cod assessment model run later in time, we show that considerably smaller annual quotas would have been advised if the true amount of cod had been known when the capelin quota was set. We discuss this fact in light of the present knowledge about recruitment success of capelin during the period. Our conclusion is that either the acoustic stock size estimates of capelin have been consistently underestimating the true stock size, the predation model overestimates the natural mortality of capelin during winter, the minimum spawning stock size considered necessary to uphold normal recruitment (B lim ) built into the harvest control rule is higher than needed to secure good recruitment, or a combination of these.

Tjelmeland, S., and Lindstrøm, U. 2005. An ecosystem element added to the assessment of Norwegian spring-spawning herring: implementing predation by minke whales. e ICES Journal of Marine Science, 62: 285e294.Predation by minke whales is incorporated in the assessment model of the Norwegian spring-spawning herring stock (SeaStar) used by the ICES Working Group. Three assessment scenarios are performed and evaluated: (1) Default historic assessment where the natural mortality (M) is fixed, (2) assessment where natural mortality is estimated both for adult and juvenile herring, (3) assessment where consumption by minke whales is modelled and the predation and residual natural mortalities are estimated. The annual consumption of juvenile herring in the Barents Sea is estimated exogenously using diet data and a bioenergetic model. The estimated consumption is included in the objective function and the parameters determining the modelled consumption are estimated together with other free parameters of the model in a single operation. The estimated total natural mortality of juvenile herring is lower than the value assumed by the working group (M Z 0.9) when either minke whales are included in the model (M Z 0.49) or the parameter is estimated directly (M Z 0.48). Assessment 3 generates 19% and 34% lower adult and juvenile stock sizes, respectively, than assessment 1, whereas assessments 2 and 3 generate relatively similar stock size estimates. The predation mortality constituted 45% and 10% of the total natural mortality of adult (M Z 0.15) and juvenile herring (M Z 0.49), respectively.