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Under the general framework of existing recruitment hypotheses, knowledge on the drivers and mechanisms involved in the determination of the year class strength of small pelagic fish (SPF) is briefly reviewed with focus on selected aspects of the adult and larval stages, related to breeding patterns, egg production, spawning habitats, reproductive potential and early life survival. An analysis of stock–recruitment time series data is carried out, showing that the maximum recruitment capacity of clupeoid stocks increases with the strength of temporal autocorrelation in recruitment (R) and decreases as the coefficient of variation of R becomes larger. Reproductive strategy in combination with the thermal and trophic conditions of the ecosystem and the life cycle pattern of the stock can influence the relative importance of high and low frequency variability in recruitment that combine to generate the population fluctuations of SPF. Selective fishing can reduce the reproductive potential and alter the spawning phenology of the stocks. To understand the ways by which the distribution, abundance and survival of larval stages are influenced by trophodynamic and physical factors, it is important to recognize all those milestones in fish ontogeny associated with significant changes in capabilities and behavior (e.g. onset of schooling). Temperature affects many parameters related to egg production and early life survival, but the relative importance of such temperature effects is expected to differ substantially in contrasting SPF habitats.

Important changes have been observed in recent decades in small pelagic fish (SPF) populations of the NW Mediterranean Sea: declines in biomass and landings of European anchovy and sardine, and a geographical expansion of round sardinella. These changes have been linked to environmental factors directly influencing annual recruitment and growth. The role of climate change in affecting the composition of plankton has also been suggested to explain declines in SPF, while other causes could be the recovery of predators, competition with other pelagic organisms that prey on early life phases of SPF (i.e. gelatinous zooplankton), interspecific competition for food, or impacts from fisheries harvest. To test the role of these potential pressures, we developed qualitative mathematical models of a NW Mediterranean pelagic food web. We used analyses of sign directed graphs and Bayesian belief networks to compare alternative hypotheses about how SPF species may have responded to combinations of different pressures. Data documenting changes in SPF populations were used to test predicted directions of change from signed digraph models. An increase in sea surface temperature (SST) that had either a positive impact on round sardinella or on gelatinous zooplankton abundance was the pressure that alone provided the most plausible insights into observed changes. A combination of various pressures, including an increase in SST, an increase of exploitation and changes to zooplankton also delivered results matching current observations. Predators of SPF were identified as the most informative monitoring variable to discern between likely causes of perturbations to populations of SPF.

Despite extensive studies of phenological shifts associated with climate change, a few unresolved issues remain. In particular, little is known about the phenological shifts of marine organisms, and simultaneous evaluations of phenological and distributional shifts in reproduction are needed. Chub mackerel Scomber japonicus in the western North Pacific is a small pelagic fish species with its main spawning ground around the Izu Islands. Since its spawning patterns are affected by the sea surface temperature (SST), changes in the spawning time and spawning ground are expected if the SST has increased around the Izu Islands. Here, to elucidate phenological and distributional shifts in reproduction associated with climate change and the underlying causes, we first examined the long-term changes in spawning patterns and spawning ground using a geostatistical model with 40 yr time series data for spawning eggs of chub mackerel in the western North Pacific. We then tested the prediction that increasing SST impacts the timing and distribution of chub mackerel reproduction. We found that the spawning period was extended owing to a delay in the end of spawning. The geographic location of the spawning ground moved northward after the 2000s, and this was probably related to the change in SST. In addition, SST in the spawning ground after the 2000s was not significantly different from the SST in the previous spawning ground. Therefore, the environmental change associated with climate change likely influences phenological and distributional shifts in reproduction simultaneously and underlines the importance of focusing on spatio-temporal changes in reproduction.

Populations of small pelagic fish (SPF) such as sardine, anchovy, herring, capelin and mackerel provide ~25% of the global annual yield of capture fisheries, and the well-being of many human coastal communities around the world, particularly in developing countries, critically depends on these SPF resources. These fishes display large ‘boom and bust’ cycles with great ecological as well socioeconomic consequences. Despite many internationally coordinated research efforts, sufficient knowledge about the drivers of SPF population dynamics and, particularly, the interactive effects of environmental and anthropogenic factors is lacking. The ecology and management of SPF were discussed in a symposium in Victoria, BC (Canada), attracting participants from 31 countries. This Theme Section consists of 22 research contributions providing fundamental insights into (1) the biology of SPF, (2) the drivers of SPF dynamics and (3) the socio economic impacts of SPF fisheries. Such insights are urgently needed for effective, ecosystem-based management of these highly variable fish populations. The symposium was an important catalyst for future, internationally coordinated research efforts to further advance our knowledge on the drivers of SPF population dynamics and the effective management of SPF fisheries.

Small and medium pelagic fish (SMPF, i.e. Scomber colias, Trachurus spp, Sardina pilchardus, and Sardinella spp) in the Canary Islands are mainly targeted by the artisanal purse-seine fleet. The waters in the archipelago (located in the coastal transition zone of the Canary Current Eastern Boundary Upwelling System) are monitored since the late nineties by a hydrographic section (RAPROCAN) designed to study the temporal variability of the eastern subtropical gyre. In this study we analyse the relationship between the SMPF abundance assumed from official sale notes (reported since 2007) and several oceanographic parameters obtained for the outermost water layer (Sea Surface Temperature, SST, and concentration of chlorophyll a, Chla) and from the 200–800 m depth waters (Sea Temperature, ST_200–800, and salinity, Salinity_200–800). Except for SST, statistically significant correlations occur between environmental variables and SMPF landings when one-year time-lag is considered, matching with the time period necessary for these species to attain legal catchable sizes and, hence, being catchable by the fishery. However, in the GLM only Chla resulted a significant explaining variable for the SMPF landings during the following year, probably because this strong correlation overshadows the ST_200–800 influence. Keeping the monitoring systems is crucial to understand, foresee and anticipate potential variations in the fishery resources and to aim the sustainable exploitation of the SMPF populations, even more challenging in the current climate change scenario.

The recent northward expansion of the round sardinella Sardinella aurita in the Mediterranean Sea has been documented as a consequence of rising sea temperature. At the same time, declines in sardine and anchovy biomass have been observed in the NW Mediterranean Sea, necessitating an assessment of whether the expansion of round sardinella may affect sardine and anchovy populations. Here, we combined stomach content and isotopic analyses to describe the trophic habits of round sardinella in the NW Mediterranean Sea and its trophic relationships with 2 sympatric small pelagic fish, European anchovy Engraulis encrasicolus and European pilchard Sardina pilchardus. Results revealed changes in the diet of round sardinella during the year. In summer, the most important prey were copepods (Acartia spp.) and cladocerans (Penilia avirostris). During winter, the diet was composed mainly of copepods and tunicates (mainly appendicularians), but microplankton was also numerically important in adult diets. In contrast to previous studies, during spring, round sardinella principally fed on salps (Thaliacea). To our knowledge, this is the first time that salps have been identified as an important prey for round sardinella. When compared to coexisting small pelagic fish, we found that round sardinella adults had a different trophic niche than anchovy and sardine. In contrast, round sardinella juveniles partially overlapped the trophic niche with the juveniles of the other 2 species. Therefore, the range expansion of round sardinella probably would not affect sardine and anchovy populations. Only in a situation of food limitation could juveniles of round sardinella compete with and affect both sympatric species. Our results provide new insights into the ecological role of this range-expanding species in the NW Mediterranean Sea, and highlight the importance of gelatinous zooplankton as prey.

This study documented the levels of microplastics in three commercially important small pelagic fish species in South African waters, namely European anchovy (Engraulis encrasicolus), West Coast round herring (Etrumeus whiteheadi) and South African sardine (Sardinops sagax). Data suggested variation between species with a higher concentration of microplastics for S. sagax (mean of 1.58 items individual-1) compared to Et. whiteheadi (1.38 items individual-1) and En. encrasicolus (1.13 items individual-1). The occurrence of microplastics was also higher for S. sagax (72%) and Et. whiteheadi (72%) compared to En. encrasicolus (57%). Microfibres accounted for 80% of ingested microplastics (the remainder were plastic fragments) with the main ingested polymers being poly(ethylene:propylene:diene) (33% occurrence), polyethylene (20%), polyamide (20%), polyester (20%) and polypropylene (7%). The abundance of ingested items was not significantly correlated with fish caudal length or body weight, and spatial investigation indicated an increase in the abundance of ingested items from the West to the South coast. Etrumeus whiteheadi is proposed as a bio-indicator for microplastics for South Africa.

The abundance and spatial occupation of European anchovyEngraulis encrasicolushave increased in the North Sea since the mid-1990s. We use a cross-disciplinary approach combining genetics, transport modelling, survey time series analyses and physical oceanographic modelling to investigate 3 hypotheses on the reasons for this change. Evidence from connectivity studies suggests that the population of North Sea anchovy is separate from that in the Bay of Biscay. The recruitment pulses observed in survey data fit a life cycle which includes spawning in early summer and larval development in late summer. This also supports the concept of population expansion originating from local remnant population(s). In terms of growth physiology, suitable thermal windows have expanded, making conditions more favourable for life cycle closure and population persistence/productivity. In addition to the increased frequency of warm summers, which favour larvae and juvenile growth, the decrease in the number of severe winters is also likely to improve overwinter survival. Overall, the evidence supports the hypothesis that the increase in anchovy abundance originated from the improved productivity of existing populations. This increase was associated with an expansion in thermal habitats and is probably not due to a northward shift in the distribution of southern conspecifics.

Essential fatty acids are suggested to be key elements in trophodynamic processes in small pelagic fish. Small pelagic fish generally acquire these fatty acids from phytoplankton and zooplankton lipids; however, little is known about the utilization characteristics of fatty acids. To clarify this, we measured the fatty acid composition of Japanese sardine, Sardinops melanostictus, during the feeding and spawning periods. A comparison of fatty acid composition between sardine and zooplankton in the feeding period indicated that sardine preferentially retained high levels of specific long-chain polyunsaturated fatty acids (LC-PUFAs), such as docosahexaenoic acid (DHA) and arachidonic acid, in the muscles compared to those in dietary organisms. Further, a comparison of sardines between the feeding and spawning periods demonstrated that the levels of these LC-PUFAs in the muscle decreased during the spawning period. Instead, the levels of LC-PUFAs in the ovary (especially DHA) increased with ovary-somatic development, indicating that in the sardine, these LC-PUFAs are transferred from the muscle to the gonad during the spawning period.

Pacific capelin Mallotus catervarius are planktivorous small pelagic fish that serve an intermediate trophic role in marine food webs. Due to the lack of a directed fishery or monitoring of capelin in the Northeast Pacific, limited information is available on their distribution and abundance, and how spatio-temporal fluctuations in capelin density affect their availability as prey. To provide information on life history, spatial patterns, and population dynamics of capelin in the Gulf of Alaska (GOA), we modeled distributions of spawning habitat and larval dispersal, and synthesized spatially indexed data from multiple independent sources from 1996 to 2016. Potential capelin spawning areas were broadly distributed across the GOA. Models of larval drift show the GOA’s advective circulation patterns disperse capelin larvae over the continental shelf and upper slope, indicating potential connections between spawning areas and observed offshore distributions that are influenced by the location and timing of spawning. Spatial overlap in composite distributions of larval and age-1+ fish was used to identify core areas where capelin consistently occur and concentrate. Capelin primarily occupy shelf waters near the Kodiak Archipelago, and are patchily distributed across the GOA shelf and inshore waters. Interannual variations in abundance along with spatio-temporal differences in density indicate that the availability of capelin to predators and monitoring surveys is highly variable in the GOA. We demonstrate that the limitations of individual data series can be compensated for by integrating multiple data sources to monitor fluctuations in distributions and abundance trends of an ecologically important species across a large marine ecosystem