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Impacts of climate change on ocean productivity sustaining world fisheries are predominantly negative but vary greatly among regions. We assessed how 39 fisheries resources-ranging from data-poor to data-rich stocks-in the North East Atlantic are most likely affected under the intermediate climate emission scenario RCP4.5 towards 2050. This region is one of the most productive waters in the world but subjected to pronounced climate change, especially in the northernmost part. In this climate impact assessment, we applied a hybrid solution combining expert opinions (scorings)-supported by an extensive literature review-with mechanistic approaches, considering stocks in three different large marine ecosystems, the North, Norwegian and Barents Seas. This approach enabled calculation of the directional effect as a function of climate exposure and sensitivity attributes (life-history schedules), focusing on local stocks (conspecifics) across latitudes rather than the species in general. The resulting synopsis (50-82°N) contributes substantially to global assessments of major fisheries (FAO, The State of World Fisheries and Aquaculture, 2020), complementing related studies off northeast United States (35-45°N) (Hare et al.,

The lack of any abundant recruiting year class of Norwegian spring-spawning (NSS) herring between 2005 and 2015 contributed to an approximate reduction of 40% in the spawning-stock biomass since 2009, i.e. from 7 to 4 million tonnes. Warming of the North Atlantic is suggested to contribute to this reduction in recruitment. In the past, a warm phase induced by a positive Atlantic Multidecadal Oscillation (AMO) in the North Atlantic was positively correlated to the NSS herring stock size. Recent unprecedented ocean warming in the Norwegian Sea ecosystem, besides elevated temperatures due to a positive AMO, seems to be outside optimal environmental conditions for early life history stages of NSS herring. We analysed 28 years of survey data using generalized additive models to reconstruct environmental conditions for drifting yolksac and preflexion stage larvae. Our results indicate that strong recruitment years were more likely when the larvae occurred simultaneously with a negative AMO during positive temperature anomalies. The transition from yolksac stages towards preflexion stages occurred while there was a slow increase in water temperature during the larval drift. Weak recruitment years generally occurred when larvae experienced elevated temperatures during the life stage transition under a positive AMO. These results augment evidence that the historical positive relationship between AMO and stock dynamics is reversed between 1988 and 2015. Albeit not implying any specific mechanistic biological interactions, we can assume that the unprecedented warming has modified the ecosystem drivers that negatively affect drifting larvae. Since 2016, the North Atlantic is shifting into a negative AMO phase, possibly resulting in the 10-year recruitment suppression of NSS herring ending soon.

Norwegian spring‐spawning herring (NSSH, Clupea harengus) is a key species in the food‐web and for fisheries in the north‐east Atlantic. NSSH has been the focus of many ecological and fisheries studies over decades and several hypotheses have been put forward to explain variations in its recruitment. We conducted an extensive literature review of the processes that have been hypothesized to control recruitment at age‐2 years. From this review, we constructed a conceptual model to represent how these processes are inter‐connected. We then evaluated several of these hypothesized processes using quantile regression modelling and the most recent available data series as input. Most of the hypotheses were not supported by our analyses. Only two hypotheses were supported: the top‐down control of herring larval stage by Atlantic mackerel (Scomber scombrus) and the positive effect of temperature on recruitment. For the latter the interpretation of the results is nevertheless ambiguous when the latest years (1998–2018) of observations are included, as the correlation then changes from positive to negative. Furthermore, when retesting the hypotheses on age‐2 years estimates, we observe a benefitting effect of a consistent strong forcing of the Norwegian Coastal Current and a possible positive effect of the NSSH spawning stock on recruitment. How much these hypotheses can be used to make predictions about future recruitment of herring remains to be tested but based on our results, the relatively short time series available and the dispersion of the observations around the regression models, we can anticipate that such predictions would have limited use for the purpose of fisheries assessment and management.

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Mortality rates in the early life-history stages of fishes are generally high yet identifying the causes remain unclear. Faltering recruitment rates of Atlantic herring (Clupea harengus) in the Norwegian Sea indicate a need to identify which mortality factors influence larval herring survival. Previous research suggests that increased predation pressure by Atlantic mackerel (Scomber scombrus) may contribute to the disconnect between spawning stock biomass and recruitment. To quantify the contribution of predation pressure by Atlantic mackerel to herring larval mortality, two research cruises were conducted within a probable “hot spot” (67–72° N) for intensified mackerel predation based on particle drift simulations. Mackerel stomach contents were analysed for herring larvae content using droplet digital polymerase chain reaction (ddPCR) with a quantitative molecular detection assay specific for herring. The ddPCR results demonstrate clear predation by mackerel on herring larvae and also suggest that the alternative use of visual examination may give misleading results. Our results show that mackerel should be considered a potentially important predator on herring larvae. The quantitative molecular assay presented here shows great promise as an efficient and specific tool to correctly identify and quantify predation pressure on early life-history stages of fishes.

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Abstract:Little is known about the concerted influence of temperature and salinity on the fecundity of clupeid fishes. Due to a globally changing climate, both physical parameters might act as stressors, severely affecting the reproductive potential of clupeid fish populations inhabiting tropical estuaries. Differences in relative batch fecundities, the gonado-somatic index, and the condition index of bonga shad (Ethmalosa fimbriata) were analysed in individual females sampled at the Senegalese coast and inside the inverse Sine Saloum estuary, where salinity increases upstream in all seasons. Multiple linear regression models on fecundity and gonadal energy storage show that clupeids can adapt towards increasing their reproductive investment at temperatures (26-30 • C) and salinities (42-51), which by far exceed marine conditions, in an effort to maximize recruitment success. This reproductive strategy, however, is accompanied by a trade-off between reproductive effort and somatic growth, which ultimately limits the species' reproductive potential inside the estuary. The observed high variability in batch fecundities might be a viable mechanism to adjust to fluctuating and rather extreme environmental conditions. Understanding the spawning biology of exploited clupeid fishes in drastically changing environments is crucial for evaluating the reproductive potential of stocks at the outer reach of their physiological performance curve.