Data from: Shifts in North Sea forage fish productivity and potential fisheries yield

Forage fish populations support large scale fisheries and are key
components of marine ecosystems across the world, linking secondary
production to higher trophic levels. While climate-induced changes in the
North Sea zooplankton community are described and documented in
literature, the associated bottom-up effects and consequences for
fisheries remain largely unidentified. We investigated the temporal
development in forage fish productivity and the associated influence on
fisheries yield of herring, sprat, Norway pout and sandeel in the North
Sea. Using principal component analysis, we analysed 40 years of
recruitment success and growth proxies to reveal changes in productivity
and patterns of synchroneity across stocks (i.e. functional
complementarity). The relationship between forage fish production and
Calanus finmarchicus (an indicator of climate change) was also analysed.
We used a population model to demonstrate how observed shifts in
productivity affected total forage fish biomass and fisheries yield. The
productivity of North Sea forage fish changed around 1993 from a higher
average productivity to lower average productivity. During the higher
productivity period, stocks displayed a covariance structure indicative of
functional complementarity. Calanus finmarchicus was positively correlated
to forage fish recruitment, however, for growth, the direction of the
response differed between species and time periods. Maximum sustainable
yield (MSY) and the associated fishing mortality (Fmsy) decreased by
33%–68% and 26%–64%, respectively, between the higher and lower
productivity periods. Synthesis and applications. The results demonstrate
that fisheries reference points for short-lived planktivorous species are
highly dynamic and respond rapidly to changes in system productivity.
Furthermore, from an ecosystem-based fisheries management perspective, a
link between functional complementarity and productivity, indicates that
ecosystem resilience may decline with productivity. Based on this, we
advise that system productivity, perhaps monitored as forage fish growth,
becomes an integral part of management reference points; in both single
species and ecosystem contexts. However, to retain social license of
biological advice when fish catch opportunities are reduced, it is crucial
that shifts in productivity are thoroughly documented and made apparent to
managers and stakeholders.