Distribution, maturity and population structure of Meganyctiphanes norvegica and Thysanoessa inermis around Iceland in spring

This study aims to explain the distribution, maturity and population structure of Meganyctiphanes norvegica and Thysanoessa inermis in springtime in relation to main hydrographic regions around Iceland: Atlantic in the southwest, Atlantic-Arctic mixture in the north and Arctic in the east. Krill were collected 14–29 May 2013 using a macrozooplankton trawl. Biomass of both species combined was significantly higher in the southwest than in north and east. M. norvegica clearly dominated in Atlantic waters, whereas T. inermis was more evenly distributed around the island, while the highest values were also observed in the southwest for this species. Simple linear regressions showed that the abundance of M. norvegica was positively related to temperature, salinity and phytoplankton concentration, while the abundance of T. inermis was negatively related to bathymetry. Multiple linear regression analyses did not add to this information of a positive relationship between abundance and temperature for M. norvegica, while T. inermis was shown to be negatively related to both temperature and bathymetry. During the latter half of May, the main spawning of both species was confined to the regions off the southwest coast. Sex ratio (males/females) of M. norvegica was higher in the southwest than in the north and east, whereas T. inermis showed a similar sex ratio all around the island. In all regions, M. norvegica appears to have a lifespan of 2 years while T. inermis of 1 year in the southwest and possibly 2 years in north and east.This work was supported by the Marine and Freshwater Research Institute (project number 15.19) and by the research programme EURO-BASIN − European Union Basin-scale Analysis, Synthesis, and Integration (FP7 contract no. 264933).Peer Reviewe

Diet and feeding strategy of Northeast Atlantic mackerel (Scombrus scomber) in Icelandic waters

Publisher's versionPredator-prey relations, as well as the trophic ecology of highly migratory marine species, is important to understand their impact on the ecosystem. Conventional methods were used to study the diet composition and feeding strategy of the Northeast Atlantic mackerel (Scombrus scomber), during their summer feeding migration to Icelandic waters in 2009–2014. In addition, generalised additive modelling (GAM) was used to determine which biological and environmental factors contribute to the variation of their stomach weight in the years 2011–2014. From the dietary analysis, we found that calanoid copepods (especially Calanus finmarchicus) were the most important contributor to the overall diet of mackerel in the years studied. Although in some years and areas, they also preyed heavily on larger prey items such as euphausiids, amphipods and megalopa larvae of crab and shrimp. The GAM showed that temperature and the time the day of sampling were significant explanatory variables for the stomach weight, while zooplankton biomass did not seem to have much influence. The Northeast Atlantic mackerel are ferocious feeders upon copepods, as well as exhibiting an overall opportunistic feeding strategy. During their feeding migration in Icelandic waters, they were found to feed on the most dominant species available to them.This research was funded by the Icelandic Centre for Research (grant number: 163371-051), the Marine and Freshwater Institute of Iceland and Matis as a part of a PhD project on the ecosystem impact of mackerel in Icelandic waters.Peer Reviewe

Otolith shape differences between ecotypes of Icelandic cod (Gadus morhua) with known migratory behavior inferred from Data Storage Tags

Otolith shape can be used to identify ecotypes of the Icelandic cod stock. The use of data storage tags (DSTs) has increased our knowledge of the stock structure of the Icelandic cod (Gadus morhua). The profiles of tagged cod reveal different migratory strategies. This has led to definition of two ecotypes within the cod stock. Frontal ecotypes reside in deep waters during feeding season and express a highly variable temperature profile associated with thermal fronts, while coastal ecotypes stay in shallow waters all year round. In this study the DSTs profiles were analysed with Cluster Analysis which revealed existence of an intermediate behaviour that expresses a variable depth profile and a feeding migration which is both shorter in time and not as deep. The main objective was to develop a morphological key based on otoliths to distinguish the ecotypes. The shape of the otoliths was extracted with shape measurements and Fast Fourier Transforms. The results of a discriminant function analysis indicated a difference in morphology between the ecotypes resulting in a successful classification

Species richness in North Atlantic fish: Process concealed by pattern

International audiencePrevious analyses of marine fish species richness based on presence‐absence data have shown changes with latitude and average species size, but little is known about the underlying processes. To elucidate these processes we use metabolic, neutral and descriptive statistical models to analyse how richness responds to maximum species length, fish abundance, temperature, primary production, depth, latitude and longitude, while accounting for differences in species catchability, sampling effort and mesh size

Relationship between atresia, fish size and condition in Icelandic cod (Gadus morhua L.)

Cod is a determinate spawner, with a high potential fecundity. That is, a large definite number of oocytes start maturing prior to spawning. Environmental factors, such as water temperature and food availability have strong effects on nutritional status and fish size. Accumulation of energy prior to spawning is important, as food intake during spawning is minimum. Fecundity is a changeable process that can be adjusted to the condition of the fish. Under favorable circumstances potential fecundity may be equivalent to the number of eggs spawned during the spawning season. When the environmental conditions are unfavorable some or all, developing oocytes included in the potential fecundity estimate may be lost through resorption from the ovary, a process known as atresia. Atresia seems mainly to be initiated under low nutritional status prior to spawning. The relationship between atresia and oocyte diameter is negatively related. As the oocyte diameter increases during the spawning season, occurrence of atresia decreases

Sensitivity of potential recruitment to stock structure in the presence of temporally varying survival.

In this paper we discuss the results-of a modelling study which quantifies differences in the number of potential recruits produced from a range of age-structured populations over a spawning season with temporally varying survival rates. We include investigations into the effects that variation in female condition and egg quality (as a function of female characteristics) may have on overall production. We also quantify the effects on production of potential recruits via changes in the overall duration of spawning seasons due to environmental influences. The population stock structure (the proportion of females in different age classes) has the most effect on overall potential recruitment. The effects of egg quality dramatically increases larger fish's output of viable offspring whilst decreasing that of smaller fish. Fish condition has a very large effect on potential recruitment, but the effects are felt by all age classes equally. The present model outputs dealing with the interactions of temporally varying survival vs. stock structure and condition suggest that the middle of the spawning period consistently produces the most potential recruits but that the relative production of recruits over the spawning season is heavily influenced by both condition and stock structure

How variable is the fecundity within and between cod stocks?

Stock reproductive potential is determined by the number of fish at age, sex ratio, proportion of mature female at age and fecundity. Fecundity data, which are fundamental in estimating egg production are not collected routinely for most fish stocks. Thus, variability in fecundity is largely ignored in the estimations of reproductive potential. The objective of this study was to identify patterns of spatial and temporal variation in the fecundity within and between different stocks of Atlantic cod (Gadus morhua) in the North Atlantic and Baltic Sea. A review of fecundity data for cod of different stocks and years was made. The examination of fecundity at size indicated a very large variability in the fecundity of cod between stocks and years. The fecundity of a 60 cm cod varied from 210 000 eggs to 2 250 000 eggs. Baltic and North Sea cod stocks were very distinct from the others, highest fecundity-at-size notwithstanding years being observed for these stocks. A different pattern of variation was observed for the other stocks. Temporal variation in fecundity appeared as important as or even more important than differences associated with stocks. Without excluding genetic differences, variations in the environmental conditions and fishing pressure could possibly explain much of the variation in the fecundity of these stocks. At a smaller scale, it also appears that the increase in the reproductive investment with age/size can differ between stocks and years. Thus, routine measurement of fecundity should be essential in estimating reproductive potential of cod