A retrospective evaluation of the Barents Sea capelin management advice

Abstract

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.

Ecosystem processes are rarely included in tactical fisheries management

“…In the Barents Sea, cod consumption of capelin has been implemented in short‐term predictions of capelin stock biomass since the 1990s (Gjøsæter et al . ). Retrospective analyses performed by Gjøsæter et al .…”

“…Retrospective analyses performed by Gjøsæter et al . () demonstrated that the cod stock predictions used for estimating consumption were consistently too small. This bias has resulted in predictions of too large capelin stocks and therefore the setting of larger quotas than prescribed by the agreed HCR.…”

From single species surveys towards monitoring of the Barents Sea ecosystem

“…Particularly, coverage of the benthic part of the ecosystem has become much better. This knowledge has yielded many scientific papers that would otherwise not have been possible (Gjøsaeter et al, 2009(Gjøsaeter et al, , 2015Fossheim et al, 2015;Jørgensen et al, 2015;Johannesen et al, 2012aJohannesen et al, , 2012bHaug et al, 2017;Kjesbu et al, 2014). However, the broadening of the data collection comes at a cost related to increased survey time and decreased spatial resolution compared to the specialized surveys that formed the basis for the BESS.…”

Harvest control rules in modern fisheries management

“…Norway has taken a number of steps to implement an ecosystem-based management scheme for the Barents Sea (Olsen et al, 2007). For example, Barents Sea capelin stock assessment scientists have for a number of years estimated cod predation (Gjøsaeter et al, 2015) and has developed at least the scientific basis for including the cod-capelin interaction in cod management. Given that capelin and cod are, typically, harvested by different fishing vessels and that Russia and Norway may value these stocks differently, it would require substantial political will to include the cod-capelin interaction in the HCR.…”

“…Recently, a new harvest control rule was introduced, again modelled after the harvest control rule for the Barents Sea capelin (Gjøsaeter et al, 2015), where empirical uncertainty is taken into account. The new rule also reflects predation on capelin that takes place between the time of the acoustic measurements of the spawning migration and the spawning season.…”