Bioenergetics modeling of the annual consumption of zooplankton by pelagic fish feeding in the Northeast Atlantic

The present study uses bioenergetics modeling to estimate the annual consumption of the main zooplankton groups by some of the most commercially important planktivorous fish stocks in the Northeast Atlantic, namely Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and NEA mackerel (Scomber scombrus). The data was obtained from scientific surveys in the main feeding area (Norwegian Sea) in the period 2005–2010. By incorporating novel information about ambient temperature, seasonal growth and changes in the diet from stomach content analyses, annual consumption of the different zooplankton groups by pelagic fish is estimated. The present study estimates higher consumption estimates than previous studies for the three species and suggests that fish might have a greater impact on the zooplankton community as foragers. This way, NEA mackerel, showing the highest daily consumption rates, and NSS herring, annually consume around 10 times their total biomass, whereas blue whiting consume about 6 times their biomass in zooplankton. The three species were estimated to consume an average of 135 million (M) tonnes of zooplankton each year, consisting of 53–85 M tonnes of copepods, 20–32 M tonnes of krill, 8–42 M tonnes of appendicularians and 0.2–1.2 M tonnes of fish, depending on the year. For NSS herring and NEA mackerel the main prey groups are calanoids and appendicularians, showing a peak in consumption during June and June–July, respectively, and suggesting high potential for inter-specific feeding competition between these species. In contrast, blue whiting maintain a low consumption rate from April to September, consuming mainly larger euphausiids. Our results suggest that the three species can coexist regardless of their high abundance, zooplankton consumption rates and overlapping diet. Accordingly, the species might have niche segregation, as they are species specific, showing annual and inter-annual variability in total consumption of the different prey species. These estimates and their inter-annual and inter-specific variation are fundamental for understanding fundamental pelagic predator-prey interactions as well as to inform advanced multispecies ecosystem models.publishedVersio

Influence of seasonal variability on the trophic structure of pelagic communities

Seasonal variations in hydrodynamic conditions play a critical role in prey availability at the base of the food web and thus have implications for trophic interactions at higher trophic levels. Here we use the combination of stomach content analysis (SCA) and stable isotope analysis (SIA) to investigate predator-prey interactions and trophic structure of the pelagic fish community of the Bay of Biscay (BoB) in spring and late summer of 2020 and 2021. We found that trophic guilds were more influenced by intra-specific affinities rather than seasonal variability. Main prey for the pelagic fish comprised a set of species belonging to different trophic positions (TPs), from low TP class Thaliacea (salps) to mid TP Engraulis encrasicolus. Vertically migrating meso- and macrozooplankton (mainly copepods and krill) also played an important role as food source for the pelagic fish community showing high trophic overlap among most of species. No differences were detected between the trophic structure in spring and late summer with fish, squids and chaetognaths at the top TPs, whereas krill, meso-zooplankton and salps had the lowest values. However, the TPs estimated using both SCA and SIA approaches were consistently lower in spring, likely due to higher densities and, thus, low TP prey availability in spring. When analyzing ontogenic changes in TPs with predator’s size, significant and positive relationships were found when TP was estimated using SIA but not when using SCA. Overall, our results reveal new insights into the seasonal trophic dynamics of the pelagic fish community of the BoB

Data from: Feeding ecology of Northeast Atlantic mackerel, Norwegian spring-spawning herring and blue whiting in the Norwegian Sea

The Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and Northeast Atlantic (NEA) mackerel (Scomber scombrus) are extremely abundant pelagic planktivores that feed in the Norwegian Sea (NS) during spring and summer. This study investigated the feeding ecology and diet composition of these commercially important fish stocks on the basis of biological data, including an extensive set of stomach samples in combination with hydrographical data, zooplankton samples and acoustic abundance data from 12 stock monitoring surveys carried out in 2005-2010. Mackerel were absent during the spring, but had generally high feeding overlap with herring in the summer, with a diet mainly based on calanoid copepods, especially Calanus finmarchicus, as well as a similar diet width. Stomach fullness in herring diminished from spring to summer and feeding incidence was lower than that of mackerel in summer. However, stomach fullness did not differ between the two species, indicating that herring maintain an equally efficient pattern of feeding as mackerel in summer, but on a diet that is less dominated by copepods and is more reliant on larger prey. Blue whiting tended to have a low dietary overlap with mackerel and herring, with larger prey such as euphausiids and amphipods dominating, and stomach fullness and feeding incidence increasing with length. For all the species feeding incidence increased with decreasing temperature, and for mackerel so did stomach fullness, indicating that feeding activity is highest in areas associated with colder water masses. Significant annual effects on diet composition and feeding-related variables suggested that the three species are able to adapt to different food and environmental conditions. These annual effects are likely to have an important impact on the predation pressure on different plankton groups and the carrying capacity of individual systems, and emphasise the importance of regular monitoring of pelagic fish diets

CTD and fish sampling data used in the ms

CTD, fishing stations, biological sampling of fish and stomach content data from Ecosystem Surveys carried out in the Norwegian Sea in May and July from 2005 to 2010

ConsNS_data

Data used to estimate annual consumption of zooplankton by small pelagic fish in the Northeast Atlantic. Bioenergetics modeling has been used to make such estimations. Therefore diet composition data, fish length & growth data (length distribution), ambient temperatura data, energy density data, stock biomass information, etc. are included

Length (<i>L</i>)–Weight (<i>W</i>) relationships used to calculate the total length at age for NEA mackerel, NSS herring and blue whiting.

<p>Length (<i>L</i>)–Weight (<i>W</i>) relationships used to calculate the total length at age for NEA mackerel, NSS herring and blue whiting.</p

Average diet composition (mean prey group mg fish^-1 weighted by the total estimated abundance per station), in percentages, for mackerel, herring and blue whiting in different water masses in May and July.

<p>All sampling years were analysed together (i.e. ‘Dataset1’ in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0149238#pone.0149238.t001" target="_blank">Table 1</a>; all stations in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0149238#pone.0149238.g001" target="_blank">Fig 1</a>). Light grey lines represent the average water mass boundaries during each season. N_st and N_f are the number of stations and fish samples, respectively. Empty stomachs were excluded from this analysis.</p

Average zooplankton biomass distribution (dry weight, g m^-2) ranged by size (<1000 μm; 1000–2000 μm; >2000 μm) in May and July.

<p>All sampling years were analysed together (i.e. ‘Dataset1’ in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0149238#pone.0149238.t001" target="_blank">Table 1</a>; all stations in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0149238#pone.0149238.g001" target="_blank">Fig 1</a>). Light grey lines represent the average boundaries for each season. N_st indicates the number of stations in each water mass.</p

Mean Pianka index of overlap in diet and percentage of co-occurrence between different predator species (paired comparisons), in May and July from 2005 to 2010.

<p>The left axes and continuous lines represent the overlap, and the right axes and dotted lines represent the percentage of co-occurrence between species. In the case of the overlap index, filled symbols indicate that the probability of observed overlap is significantly lower than expected (based on 1000 simulated replicates), i.e., p(obs ≤ exp) < 0.05 (see section 2.5 for details). For herring–blue whiting (July) panel (lower left), the single co-occurrence measurement is indicated with a cross.</p