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Previous studies have shown that four commercially important demersal species, namely Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and European plaice (Pleuronectes platessa), spawn in distinct areas across the North Sea. Based on two comprehensive ichthyoplankton surveys in 2004 and 2009, the present study uses generalized additive mixed models to delimit these spawning grounds using the distribution of recently spawned eggs, investigates their relationship to specific environmental conditions, and examines egg dispersal during their development. Results indicate that presence-absence of early stage eggs is more related to temporal and topographic variables, while egg densities are closely linked with hydrography. Egg distribution patterns were relatively consistent during development and only changed near hatching. Compared with historic observations, the location of the spawning grounds appeared stable on the broad scale but centres of egg abundance varied between the surveyed years. Potential effects of long-term climate change and anthropogenic short-term disturbances, such as seismic surveys, on fish reproduction are discussed, pointing out the demand for multispecies studies on these issues. Des etudes anterieures ont demontre que quatre especes demersales d'importance commerciale, soit la morue franche (Gadus morhua), l'aiglefin (Melanogrammus aeglefinus), le merlan (Merlangius merlangus) et la plie (Pleuronectes platessa), fraient dans des aires distinctes dans la mer du Nord. A la lumiere de deux inventaires exhaustifs de l'ichtyoplancton realises en 2004 et 2009, l'etude fait appel a des modeles additifs generalises mixtes pour delimiter ces lieux de frai sur la base de la repartition des oeufs recemment pondus, explore leur lieu avec differentes conditions ambiantes et examine la dispersion des oeufs durant leur developpement. Les resultats indiquent que la presence ou l'absence d'oeufs precoces est plus fortement reliee a des variables temporelles et topographiques, alors que les densites d'oeufs sont etroitement reliees a l'hydrographie. Les motifs de repartition des oeufs etaient assez constants durant le developpement et ne changeaient qu'a l'approche de l'eclosion. Comparativement aux observations passees, l'emplacement des lieux de frai semblait stable a grande echelle, mais l'emplacement de centres d'abondance d'oeufs etait different pour les deux annees d'inventaire. Les effets potentiels de changements climatiques a long terme et de perturbations d'origine humaine de courte duree, comme des leves sismiques, sur la reproduction des poissons sont abordes, soulignant la pertinence d'etudes plurispecifiques sur ces questions. [Traduit par la Redaction], Introduction Annually visited spawning grounds are a common feature in the life cycle of many fish populations (Daan et al. 1981; Heath et al. 1994) and have been demonstrated for [...]

Among all human-induced pressures, ocean warming is expected to be one of the major drivers of change in marine ecosystems. Fish species are particularly vulnerable during embryogenesis. Here, the impact of temperature was assessed on embryonic stages of Atlantic herring (Clupea harengus), a species of high socio-economic interest, with a particular focus on the under-studied eastern English Channel winter-spawning component (Downs herring). Key traits linked to growth and development were experimentally evaluated at three temperatures (8°C, 10°C and 14°C), from fertilization to hatching, in standardized controlled conditions. Overall negative impacts of increased temperature were observed on fertilization rate, mean egg diameter at eyed stage, hatching rate and yolk sac volume. A faster developmental rate and a change in development stage frequency of newly hatched larvae were also observed at higher temperature. Potential parental effects were detected for four key traits (i.e. fertilization rate, eyed survival rate, mean egg diameter and hatching rate), despite a limited number of families. For instance, a large variability among families was shown in survival rate at eyed stage (between 0 and 63%). Potential relationships between maternal characteristics and embryo traits were therefore explored. We show that a substantial proportion of variance (between 31 and 70%) could be explained by the female attributes considered. More particularly, age, traits linked to life history (i.e. asymptotic average length and Brody growth rate coefficient), condition and length were important predictors of embryonic key traits. Overall, this study constitutes a stepping-stone to investigate potential consequences of warming on Downs herring recruitment and provides first insights on potential parental effects. [ABSTRACT FROM AUTHOR]

Condition indices aim to evaluate the physiological status of fish larvae by estimating both the level of starvation and potential of survival. Histological indices reveal direct effects of starvation whereas biochemical indices such as lipid classes or RNA:DNA ratios are used as proxies of condition, giving information on the amount of energy reserves and growth rate, respectively. We combined these three indices to evaluate ontogenetic variations of growth performance, lipid dynamics and nutritional condition of plaice larvae caught in the field during winter 2017 in the eastern English Channel and the Southern Bight of the North Sea. RNA:DNA ratios showed that larvae at the beginning of metamorphosis (stage 4) had a lower growth rate than younger individuals (stages 2 and 3). A significant increase in the proportion of triglycerides also occurred at stage 4, indicating energy storage. Histological indices indicated that most of the larvae were in good condition, even younger ones with low lipid reserves. There was, however, an increase in the proportion of healthy individuals over ontogeny, especially with respect to liver vacuoles which were larger and more numerous for stage 4 larvae. Combined together, these condition indices revealed the ontogenetic shift in the energy allocation strategy of plaice larvae. Young larvae (stages 2 and 3) primarily allocate energy towards somatic growth. The decrease in growth performance for stage 4 was not related to poor condition, but linked to a higher proportion of energy stored as lipids. Since the quantity of lipid reserves is particularly important for plaice larvae to withstand starvation during metamorphosis, this could be considered as a second critical period after the one of exogenous feeding for larval survival and recruitment success. [ABSTRACT FROM AUTHOR]

Dusky rockcod, Trematomus newnesi , is a widely distributed neritic circumpolar Antarctic fish species. We conducted a study on age and growth of T. newnesi in coastal waters of Adélie Land in East Antarctica. A total of 289 specimens were collected in 2003, 2005, 2006 and 2009. They consisted of 122 females, 132 males and 35 immature specimens. Total length (TL) and total weight (W) of these fish ranged from 13.5 to 25 cm and 19.7–174 g respectively for females and from 12 to 20.9 cm and from 24.1 to 144.1 g for males. The TL/W relationship was described by the following parameters: a = 7.2.10 −3 and b = 3.127, showing no significant difference between sex (ANCOVA, P < 0.05). Fish age was estimated by counting annual growth increments on polished transverse sections of sagittal otoliths. Age estimates varied from 3 to 14 years. There was a significant relationship between otolith morphological features (weight and radius ) and age with no difference between males and females (p > 0.05). The estimated values of Von Bertalanffy growth curve L ∞ (cm), W ∞ (g) and k were 26.6, 200.6 and 0.13 for females and 24.5, 147.0 and 0.15 for males respectively. The indices of growth performance between sexes were not significant. However, potential difference in growth rate between the morphs cannot be neglected. [ABSTRACT FROM AUTHOR]

Spatial interactions between saithe (Pollachius virens) and hake (Merluccius merluccius) were investigated in the North Sea. Saithe is a well-established species in the North Sea, while occurrence of the less common hake has recently increased in the area. Spatial dynamics of these two species and their potential spatial interactions were explored using binomial generalized linear models (GLM) applied to the International Bottom Trawl Survey (IBTS) data from 1991 to 2012. Models included different types of variables: (i) abiotic variables including sediment types, temperature, and bathymetry; (ii) biotic variables including potential competitors and potential preys presence; and (iii) spatial variables. The models were reduced and used to predict and map probable habitats of saithe, hake but also, for the first time in the North Sea, the distribution of the spatial overlap between these two species. Changes in distribution patterns of these two species and of their overlap were also investigated by comparing species' presence and overlap probabilities predicted over an early (1991–1996) and a late period (2007–2012). The results show an increase in the probability over time of the overlap between saithe and hake along with an expansion towards the southwest and Scottish waters. These shifts follow trends observed in temperature data and might be indirectly induced by climate changes. Saithe, hake, and their overlap are positively influenced by potential preys and/or competitors, which confirms spatial co-occurrence of the species concerned and leads to the questions of predator–prey relationships and competition. Finally, the present study provides robust predictions concerning the spatial distribution of saithe, hake, and of their overlap in the North Sea, which may be of interest for fishery managers. [ABSTRACT FROM PUBLISHER]

Abstract: This paper investigates the abundance and distribution of Pleuragramma antarcticum larvae by size class in the Dumont d’Urville Sea from 2004 to 2010. Samples were collected between Dumont d’Urville station and the Mertz Glacier Tongue onboard the RV l’Astrolabe for studying the inter-annual and spatial distribution of fish larvae and the TRV Umitaka Maru for looking at life stages vertical distributions. The seabed depression adjacent to the Mertz Glacier Tongue and in Commonwealth Bay hosted high abundances of small P. antarcticum larvae, while larger larvae were found in lower abundance and further offshore. We found that canyons, sea ice, stability of the water column and temperatures are important features for determining suitable areas for young larvae. [Copyright &y& Elsevier]

The processes that control the spatial distribution of North Sea whiting ( Merlangius merlangus) spawning adults are investigated using a statistical multi-model approach. Models of external and internal controls on the population, such as environmental conditions, spatial constraints, present or past spatial distribution, and demographic state of the population, are evaluated, compared and ranked to select those that are the best able to predict the observed distribution of spawning adults. Model selection is greatly influenced by the selection method, either based on data fitting or prediction, as well as by the threshold value used to stop the selection. Model selection based on prediction tends to select simpler models than selection based on data fitting. The hypotheses underlying the selected models are inferred to play a significant role in controlling North Sea whiting spatial distribution. The multi-model inference approach developed in this study enables comparison of several theoretical concepts and hypotheses and the results provide important clues on the processes involved in the control of the spatial distribution of whiting. We conclude that whiting has a high spatial fidelity to spawning site which can be linked to either geographical attachment or year-to-year persistence of the spatial distribution of the population. Environmental factors - temperature and salinity - appear to influence the geographical extent of spawning whiting distribution, whereas local abundance levels are primarily controlled by internal factors, i.e., population size and spatial segregation between ages. [ABSTRACT FROM AUTHOR]

Understanding and predicting the distribution of organisms in heterogeneous environments lies at the heart of ecology. The spatial distribution of fish populations observed in the wild results from the complex interactions of multiple controls both external or internal to the fish populations. Whilst species distribution models (SDMs) have been mostly concerned with static description of species distribution as a function of environmental constraints, models of animal movements (MAMs) have focussed on the dynamic nature of spatial distribution of groups of individuals under a number of constraints external and internal to the population. Besides SDMs and MAMs, modelling the spatial distribution of fish populations can be achieved by models that are fundamentally static (like SDMs) but can also incorporate many hypotheses on the control of fish spatial distribution (like MAMs). The hypotheses underlying these models need to make sense at the population level - rather than at the individual or species level -we term these 'population distribution models' (PDMs). PDMs are statistical models that rely on several hypotheses, which include: (i) control through geographical attachment, (ii) environmental conditions, (iii) density-dependent habitat selection, (iv) spatial dependency, (v) population demographic structure, (vi) species interactions and (vii) population memory. We review the basis behind each of these conceptual models and we examine corresponding numerical applications. We argue that the conceptual models are complementary rather than competing, that existing numerical applications are still rarely compared and combined, and that PDMs can offer a statistical framework to achieve this. We recommend that the numerical models associated with different hypotheses be constructed within such a common general framework. This will permit evaluation, comparison and combination of the multiple hypotheses on fish spatial distribution. It will ultimately lead to a more comprehensive understanding of the factors controlling the spatial distribution of fish populations and to more accurate predictions in which model uncertainty is accounted for. [ABSTRACT FROM AUTHOR]

Abstract Protozooplankton (PZP) (here size range: 12–200 μm) are rarely sampled over a broad scale, especially in ecosystem monitoring programs, despite their trophodynamic importance as grazers in the microbial loop and as prey for larger zooplankton and early life stages of fish. In this study we sampled PZP from Dutch, French, German and Norwegian research vessels taking part in the annual ICES coordinated International Bottom Trawl Survey (IBTS) which provides data on fish stock abundances and status for the entire North Sea. The abundance, biomass, composition and distribution of PZP were examined at 39 stations across the North Sea (from 3.2°W to 7.6°E and 50.5 to 59.8°N) in mid-winter (January–February 2014), a period of the year which is under-investigated so far. Twenty four taxa of dinoflagellates and ciliates were identified. Two groups comprised 89% of the total abundance of PZP: Gymnodinium spp. and other athecate dinoflagellates (68%) and Strombidium spp. and other naked ciliates (21%). The biomass of PZP at each station ranged between 0.08 and 2.4 μg C L−1, which is much lower than that reported for spring or summer (≥100 μg C L−1) in the North Sea. Relatively small-sized (<40 μm) PZP contributed 46% of the total biomass. No significant spatial pattern in the composition of the PZP community was found, although the total abundance of tintinnids was highest in the southern North Sea, an important over-wintering area for marine fish larvae. Using this fish survey (IBTS) as a sampling platform allowed us to obtain a synoptic view of the PZP community over a large area. The present collaborative effort provides an example of how existing monitoring platforms can be augmented in the future to collect relevant data and potential ecological indicators needed to advance the ecosystem-based approach to managing marine systems. Highlights • Winter protozooplankton community examined using a routine fisheries survey. • Homogenous spatial distribution of the protozooplankton community composition • Small sized cells (<40 μm) dominated the system. • PZP community mainly composed of athecate dinoflagellates and aloricate ciliates. • Highest protozooplankton biomass found towards southern North Sea areas. [ABSTRACT FROM AUTHOR]

This study aims to characterize the larval feeding strategy of the Downs sub-population of North Sea herring ( Clupea harengus L.). Diet composition, vacuity rate and prey selectivity of larvae from 8 to 15 mm collected during the International Bottom Trawl Survey (IBTS) - MIK sampling from 2008 to 2013 were assessed by direct observation of their gut contents using Scanning Electron Microscopy (SEM). The high contribution of protists and small zooplanktonic prey observed in the gut contents proved the relevance of SEM to study the diet of first feeding larvae. The relatively low vacuity rate of 45% suggests that food may not be a limiting factor for Downs herring larvae in winter. These larvae appeared to be omnivorous and there was a clear shift in term of prey composition at a size of 13 mm. Smaller larvae (8–12 mm) fed on a higher diversity of small prey, mainly small copepods ( Oncaea spp. and Euterpina acutifrons ), invertebrate eggs, diatoms ( Psammodicthyon panduriforme and Coscinodiscus spp.) and dinoflagellates ( Dinophysis acuminate and Prorocentrum micans ) whereas bigger larvae (13–15 mm) fed on a lower diversity of larger prey, mainly copepods ( Temora longicornis and Paracalanus parvus ) and dinoflagellates ( Gonyaulax spp.). Downs herring larvae had clear prey preferences as some dinoflagellates ( Pyrophacus spp., Gonyaulax spp., P. micans and Porocentrum lima) , invertebrate eggs, copepods ( Oncaea spp. and nauplii) and diatoms ( Thalassiosira curviseriata ) were positively selected and other diatoms ( Nitzschia spp., Thalassiosira tenera , Thalassiosira spp. and Chaetoceros spp.) and copepods ( Pseudocalanus elongatus , T. longicornis and Unidentified calanoid) were negatively selected. We argue that this shift in term of prey preferences occurring at a size of 13 mm constitutes the critical period for Downs herring larvae. [ABSTRACT FROM AUTHOR]

Acoustic data collected during fishing activities has been used to obtain nightly biomass and exploitation rate estimates for Atlantic herring (Clupea harengus) spawning components. Beginning in 2002, fishery based acoustic data were collected on the five major fall spawning aggregations in the sGSL and nightly biomass was estimated. Derivation of an annual seasonal index of herring biomass from fishery nightly biomass estimates have been problematic because of uncertainties associated with the behaviour of herring which accumulate prior to spawning, as well as a significant proportion of missing observations. We have developed a model which addresses these issues using nightly biomass estimates derived from acoustic data collected between 2002 and 2012 from the Fisherman's Bank fall spawning component gillnet fishery in the Southern Gulf of St. Lawrence. We conclude that these estimates identify annual changes in biomass and exploitation rate in this fishery. The method is generally applicable where the fishermen's search pattern covers the range of available fish schools during a fishing trip. Using inshore fishery biomass estimated from acoustic data to derive annual stock assessment indices could lead to the introduction of the proven reserve concept and empirical reference points to be incorporated into fishery management decisions, thus making the advisory process more transparent and responsive to local herring abundance. [ABSTRACT FROM AUTHOR]