Gudrun Marteinsdottir

Evidence of Segregated Spawning in a Single Marine Fish Stock: Sympatric Divergence of Ecotypes in Icelandic Cod?

There is increasing recognition of intraspecific diversity and population structure within marine fish species, yet there is little direct evidence of the isolating mechanisms that maintain it or documentation of its ecological extent. We analyzed depth and temperature histories collected by electronic data storage tags retrieved from 104 Atlantic cod at liberty ≥1 year to evaluate a possible isolating mechanisms maintaining population structure within the Icelandic cod stock. This stock consists of two distinct behavioral types, resident coastal cod and migratory frontal cod, each occurring within two geographically distinct populations. Despite being captured together on the same spawning grounds, we show the behavioral types seem reproductively isolated by fine-scale differences in spawning habitat selection, primarily depth. Additionally, the different groups occupied distinct seasonal thermal and bathymetric niches that generally demonstrated low levels of overlap throughout the year. Our results indicate that isolating mechanisms, such as differential habitat selection during spawning, might contribute to maintaining diversity and fine-scale population structure in broadcast-spawning marine fishes.

Genetic structure of wild and reared Atlantic salmon (Salmo salar L.) populations in Iceland

to five (10.4%) in the farmed fish (Isno). Mean polymorphism (P) was 6.5%. Seven variable enzyme loci were observed in this study: AAT-4*, GLUDH-2*, GPI-3*, IDDH-2*, IDHP-3*, MDH-3* and MEP-2*. Average observed heterozygosity (H) varied from 0.022 (s.e.=0.014) to 0.048 (s.e.=0.023) and the mean H was 0.030. Heterogeneity of allele frequencies of sample years and age groups within and between the rivers and drainages in Iceland is presented. Wright’s F,, value was 0.062 and the total gene diversity (Hr) was 0.048. Nei’s genetic distance value (D) between salmon from the 32 rivers varied from 0.000 to 0.006, with a mean value of 0.0017. Genetic distance values between the salmon from the rivers and the farmed salmon varied from 0.000 to 0.008 (mean D=0.0032). The majority of salmon rivers in Iceland contain genetically distinct populations and in larger river systems there may be more than one population. Salmon populations within the same region showed lower values of genetic distance than between populations in different regions. These findings have important management implications. Every effort should be taken to avoid genetic mixing and consequent breakdown of stock differentiation causing destruction of the local adaptations in wild stocks. Salmon enhancement should, therefore, be conducted on a river stock basis, as has been the practice in Iceland for the last 10 to 15 years. Ocean ranching stations should use stocks from the local region, the size of the operation should be carefully considered and measures should be taken to minimize straying. Cage farming of salmon now only occurs in a few locations in Iceland and in most cases these are at a distance from salmon rivers. However, use of local stocks and sterile fish should be seriously considered. 0 1997 International Council for the Exploration of the Sea

Spatial partitioning of relative fishing mortality and spawning stock biomass of Icelandic cod

Abstract Fishing mortality affects demographic characteristics of a population, and is a major determinant in loss of spawners from a fish stock; directly influencing stock productivity and recruitment variability. We investigated the effects of fishing on the composition of the spawning stock of cod (Gadus morhua) in Icelandic waters by partitioning relative fishing mortality and spawning stock biomass (SSB) into finer spatial components than those used previously. Geographic regions of reproductive importance were identified around the country, in contrast to traditional paradigms of cod stock structure in Icelandic waters. SSB and relative fishing mortality were unevenly distributed in waters around Iceland, where gill nets selectively removed larger spawners from inshore waters of the main spawning grounds, whose demographic characteristics were more conducive to progeny survival. Changes in stock structure and demographic characteristics caused by changes in spatially explicit exploitation patterns can significantly affect stock productivity through differential loss of reproductive potential. Our approach of partitioning SSB into finer spatial units than those used previously, enabled a more detailed analysis of the distribution of spawners and exploitation of the Icelandic cod stock, and will enable the development of spatially disaggregated models of total egg production, that in turn may assist in differentiating the inherent variability within stock–recruitment relationships.

Contrasting trends in two condition indices: bathymetric and spatial variation in autumn condition of Icelandic cod Gadus morhua.

Bathymetric and regional variation in condition of Icelandic cod Gadus morhua in autumn is compared to that previously observed in spring. Once again, contradicting patterns in the hepato-somatic index and a morphometric index of fish condition were observed. The relevance of this persistent spatial variation in condition in relation to emerging evidence of behavioural types is discussed.

Characterization of Atlantic Cod Spawning Habitat and Behavior in Icelandic Coastal Waters

The physical habitat used during spawning may potentially be an important factor affecting reproductive output of broadcast spawning marine fishes, particularly for species with complex, substrate-oriented mating systems and behaviors, such as Atlantic cod Gadus morhua. We characterized the habitat use and behavior of spawning Atlantic cod at two locations off the coast of southwestern Iceland during a 2-d research cruise (15–16 April 2009). We simultaneously operated two different active hydroacoustic gear types, a split beam echosounder and a dual frequency imaging sonar (DIDSON), as well as a remotely operated underwater vehicle (ROV). A total of five fish species were identified through ROV surveys: including cusk Brosme brosme, Atlantic cod, haddock Melanogrammus aeglefinus, lemon sole Microstomus kitt, and Atlantic redfish Sebastes spp. Of the three habitats identified in the acoustic surveys, the transitional habitat between boulder/lava field and sand habitats was characterized by greater fish density and acoustic target strength compared to that of sand or boulder/lava field habitats independently. Atlantic cod were observed behaving in a manner consistent with published descriptions of spawning. Individuals were observed ascending 1–5 m into the water column from the bottom at an average vertical swimming speed of 0.20–0.25 m s−1 and maintained an average spacing of 1.0–1.4 m between individuals. Our results suggest that cod do not choose spawning locations indiscriminately despite the fact that it is a broadcast spawning fish with planktonic eggs that are released well above the seafloor.

Spatial and temporal variation in the distribution and density of pelagic fish species in Lake Victoria, East Africa

The distribution and densities of three pelagic fish taxa (Nile Perch, Lates niloticus, Rastrineobola argentea [Dagaa], and the haplochromine cichlids) in Lake Victoria were estimated through 17 lake-wide acoustic surveys conducted in two series (August 1999–August 2002 and August 2005–September 2011). Nile Perch densities were estimated through echo-counting, while Dagaa and haplochromines by echo-integration. Mixed generalized linear model estimates indicated up to 30% decline in Nile Perch densities in the deep and coastal areas and up to 65% reduction in the shallow inshore areas over the study period. There was a twofold increase in Dagaa densities and a 10% increase in haplochromines. The distribution and densities of these species were influenced by season, stratum and year of survey. In addition to fish exhibiting seasonal clustering in the upper layers of the water column, they also appeared to spread to shallow inshore waters. The Nyanza, Speke, and Emin Pasha Gulfs demonstrated diverse spatial stock oscillations in abundance, and distribution which call for a need to include area disaggregated considerations in stochastic models when predicting dynamics of fish stocks.

Identification of stock components using morphological markers.

This study investigated the development of a quantitative method for distinguishing stock components of Icelandic cod Gadus morhua based on visual examination of morphology. The stock is known to be structured into genetically distinct geographic components (north and south of Iceland) and behavioural types that spawn sympatrically. Differences in morphology were tested between locations, genotypes (a proxy for behaviour) and sexes. Results show morphological markers on the head, fins and body of G. morhua that are correlated with the sex, genotype of the fish at the pantophysin (pan-I) locus and the location at which the fish were caught. Females were found to have relatively deep bodies, and the pan-I(BB) genotype (associated with deep-water feeding behaviour) have greater gaps between their fins. Overall, morphology is more useful for distinguishing sympatric genotypes but less powerful at identifying genetically distinct geographic sub-populations, perhaps because counter-gradient evolution reduces phenotypic differences even with an underlying genetic cause.

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.

Abundance and distribution of early life stages of krill around Iceland during spring

ABSTRACT Abundance, distribution and development of early life stages of krill (eggs, nauplii, calyptopes and furciliae) around Iceland were studied during the latter half of May 2013. Multivariate analyses were used to examine the relationships between water mass characteristics and phytoplankton spring bloom dynamics and distribution of krill. The results show that krill eggs, nauplii and calyptopes were most abundant over the shelf edges off the southwest and east coasts, while furciliae were most abundant on the shelf off the southwest coast. Meganyctiphanes norvegica and Thysanoessa longicaudata larvae were found mainly in the southwest, while T. inermis larvae were found in highest numbers on the east coast. Redundancy analysis showed that phytoplankton biomass, temperature and bottom depth explained 41% of the distribution pattern of early ontogenetic krill stages. In areas where krill eggs and larvae were most abundant (off the southwest coast), the phytoplankton spring bloom was in an advanced state, and the phytoplankton biomass and temperature were particularly high.

Evidence from data storage tags for the presence of lunar and semi-lunar behavioral cycles in spawning Atlantic cod

Understanding the environmental processes determining the timing and success of reproduction is of critical importance to developing effective management strategies of marine fishes. Unfortunately it has proven difficult to comprehensively study the reproductive behavior of broadcast-spawning fishes. The use of electronic data storage tags (DSTs) has the potential to provide insights into the behavior of fishes. These tags allow for data collection over relatively large spatial and temporal scales that can be correlated to predicted environmental conditions and ultimately be used to refine predictions of year class strength. In this paper we present data retrieved from DSTs demonstrating that events putatively identified as Atlantic cod spawning behavior is tied to a lunar cycle with a pronounced semi-lunar cycle within it. Peak activity occurs around the full and new moon with no evidence of relationship with day/night cycles.