Atlantic Cod Gadus Morhua Research Articles

Spatial, temporal, and environmental influences on Atlantic cod Gadus morhua offshore recruitment signals in Newfoundland

Numerous studies demonstrate the utility of information from coastal seine surveys for monitoring juveniles of Atlantic cod Gadus morhua, but few studies have linked such surveys to older ages within cohorts. We related juvenile (age-0 and -1) cod population components at a long-term monitoring site in Newfoundland to offshore pre-adult (age-3) cod recruitment at multiple spatial scales and explored some environmental and biological factors that affect juvenile-recruit relationships. Our models revealed significant relationships between juvenile and pre-adult abundance. The strength of these relationships varied with distance from nursery habitats and among fisheries management zones. Additionally, chlorophyll a concentration and body length during early life stages appeared to influence the strength of the relationship between juvenile and age-3 abundance. The potential to use juveniles as general indicators of future pre-adult abundance can aid in planning for low recruitment years and improve inferences about the response of cod population abundance to environmental changes. This study contributes to the growing body of knowledge demonstrating the utility of juvenile surveys in anticipating future year-class strength.

Impact of salmon farming on Atlantic cod spatio-temporal reproductive dynamics

Salmon farming in marine net pens is a major activity in many temperate regions. This industry may affect coastal ecosystems in several ways, such as with waste pollution and parasite spillover. Less is known about the extent to which salmon farming disrupts the use of inshore spawning grounds by wild fish, such as the Atlantic cod Gadus morhua. Acoustic telemetry was therefore used to explore cod space use during the spawning season in a coastal region in mid-Norway with multiple salmon farms. Acoustic receivers were placed in clusters at 5 known cod spawning grounds and 6 nearby salmon farms. Data from 481 adult cod caught at the spawning grounds during 2017-2019 and equipped with acoustic telemetry transmitters were analysed. Overall, fewer fish were detected at farms than spawning grounds, even when accounting for distance from release point. Individual cod residency (days detected / duration of spawning period) was generally higher at the spawning grounds close to farms but low at the farms themselves, with little apparent spawning at the farm localities. In contrast, spawning was clearly occurring at the nearby spawning grounds, with cod spending weeks (n = 316) or months (n = 158) there during the spawning period. Males had longer residence times at spawning grounds than females, likely linked to the cod mating system. Overall, we found little support for the assertion that salmon farms disrupt inshore spawning dynamics of cod using nearby spawning grounds presently, either by attracting spawners to farms or causing fish to leave these grounds.

Resource-driven colonization by cod in a high Arctic food web.

Climate change is commonly associated with many species redistributions and the influence of other factors may be marginalized, especially in the rapidly warming Arctic.The Barents Sea, a high latitude large marine ecosystem in the Northeast Atlantic has experienced above‐average temperatures since the mid‐2000s with divergent bottom temperature trends at subregional scales.Concurrently, the Barents Sea stock of Atlantic cod Gadus morhua, one of the most important commercial fish stocks in the world, increased following a large reduction in fishing pressure and expanded north of 80°N.We examined the influence of food availability and temperature on cod expansion using a comprehensive data set on cod stomach fullness stratified by subregions characterized by divergent temperature trends. We then tested whether food availability, as indexed by cod stomach fullness, played a role in cod expansion in subregions that were warming, cooling, or showed no trend.The greatest increase in cod occupancy occurred in three northern subregions with contrasting temperature trends. Cod apparently benefited from initial high food availability in these regions that previously had few large‐bodied fish predators.The stomach fullness in the northern subregions declined rapidly after a few years of high cod abundance, suggesting that the arrival of cod caused a top‐down effect on the prey base. Prolonged cod residency in the northern Barents Sea is, therefore, not a certainty.

Accounting for non-stationary stock–recruitment relationships in the development of MSY-based reference points

Asbtract Stock–recruitment relationships (SRRs) may vary over time due to ecological and anthropogenic impacts, challenging traditional approaches of calculating maximum sustainable yield (MSY)-based reference points that assume constant population traits. We compare seven methods to calculate MSY, FMSY and BMSY by modelling constant, stochastic (uncorrelated), and autocorrelated SRRs using simulations and two case studies [Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) on the Grand Bank off Newfoundland, Canada]. Results indicated that the method used to model SRRs strongly affected the temporal pattern of recruitment projection, and the variations generated by autocorrelated SRRs were more similar to observed patterns. When the population productivity had low-frequency and large-magnitude variations, stochastic SRRs generated greater MSY and FMSY estimates than constant or autocorrelated SRRs, while no consistent pattern of BMSY was detected. In the case studies, stochastic and autocorrelated SRRs produced asymmetric relationships between fishing mortality and yield, with higher risk of overfishing by going beyond FMSY. Overall, our results suggest that caution should be taken when calculating MSY-based reference points in highly dynamic ecosystems, and correctly accounting for non-stationary population dynamics could, therefore, lead to more sustainable fisheries.

Utilizing next-generation sequencing to identify prey DNA in western North Atlantic grey seal Halichoerus grypus diet

Increasing grey sealHalichoerus grypusabundance in coastal New England is leading to social, political, economic, and ecological controversies. Central to these issues is the foraging ecology and diet composition of the seals. We studied grey seal feeding habits through next-generation sequencing of prey DNA using 16S amplicons from seal scat (n = 74) collected from a breeding colony on Monomoy Island in Massachusetts, USA, and report frequency of occurrence and relative read abundance. We also assigned seal sex to scat samples using a revised PCR assay. In contrast to current understanding of grey seal diet from hard parts and fatty acid analysis, we found no significant difference between male and female diet measured by alpha and beta diversity. Overall, we detected 24 prey groups, 18 of which resolved to species. Sand lanceAmmodytesspp. were the most frequently consumed prey group, with a frequency of occurrence (FO) of 97.3%, consistent with previous studies, but Atlantic menhadenBrevoortia tyrannus, the second most frequently consumed species (FO = 60.8%), has not previously been documented in US grey seal diet. Our results suggest that a metabarcoding approach to seal food habits can yield important new ecological insights, but that traditional hard parts analysis does not underestimate consumption of Atlantic codGadus morhua(FO = 6.7%, Gadidae spp.) and salmonSalmo salar(FO = 0%), 2 particularly valuable species of concern.

Estimating the Discard Mortality of Atlantic Cod in the Southern Gulf of Maine Commercial Lobster Fishery

AbstractThe Gulf of Maine (GOM) commercial lobster fishery has approximately 3.5 million actively fished traps and captures several nontargeted groundfish species, including Atlantic Cod Gadus morhua, as bycatch, yet there has been limited research on the incidental mortality of groundfish in this fishery. Although the mortality of Atlantic Cod has been estimated in other GOM commercial fisheries, unaccounted discard mortality in the lobster fishery may impair recovery efforts for this stock. To help meet research needs, we assessed the discard mortality rate of Atlantic Cod captured in the Maine Lobster Management Zone G commercial lobster fishery using acoustic transmitters and observations of viability. From 2016 to 2017, 111 Atlantic Cod were captured in 18,853 individual trap hauls and were observed for viability. A subsample of 54 Atlantic Cod was externally tagged with acoustic transmitters and observed after release. The combined at‐vessel mortality (9.3%) and model‐based long‐term discard mortality (17.1%) estimates indicated an overall discard mortality rate of 24.8% for Atlantic Cod captured in commercial lobster gear. Based on this finding and the low bycatch of Atlantic Cod in the lobster fishery, the commercial lobster fishery may not be responsible—to the extent previously assumed—for hindering the GOM Atlantic Cod stock’s regrowth.

Genomic analysis reveals neutral and adaptive patterns that challenge the current management regime for East Atlantic cod Gadus morhua L.

Challenging long‐held perceptions of fish management units can help to protect vulnerable stocks. When a fishery consisting of multiple genetic stocks is managed as a single unit, overexploitation and depletion of minor genetic units can occur. Atlantic cod (Gadus morhua) is an economically and ecologically important marine species across the North Atlantic. The application of new genomic resources, including SNP arrays, allows us to detect and explore novel structure within specific cod management units. In Norwegian waters, coastal cod (i.e. those not undertaking extensive migrations) are divided into two arbitrary management units defined by ICES: one between 62° and 70°N (Norwegian coastal cod; NCC) and one between 58° and 62°N (Norwegian coastal south; NCS). Together, these capture a fishery area of >25,000 km2 containing many spawning grounds. To assess whether these geographic units correctly represent genetic stocks, we analysed spawning cod of NCC and NCS for more than 8,000 SNPs along with samples of Russian White Sea cod, north‐east Arctic cod (NEAC: the largest Atlantic stock), and outgroup samples representing the Irish and Faroe Sea's. Our analyses revealed large differences in spatial patterns of genetic differentiation across the genome and revealed a complex biological structure within NCC and NCS. Haplotype maps from four chromosome sets show regional specific SNP indicating a complex genetic structure. The current management plan dividing the coastal cod into only two management units does not accurately reflect the genetic units and needs to be revised. Coastal cod in Norway, while highly heterogenous, is also genetically distinct from neighbouring stocks in the north (NEAC), west (Faroe Island) and the south. The White Sea cod are highly divergent from other cod, possibly yielding support to the earlier notion of subspecies rank.

Improving the age reading of East Greenland Atlantic cod (Gadus morhua) by determining otolith growth zone timing and annuli widths

AbstractAccurate and consistent age readings are cornerstones of age‐based assessment and management of exploited fish stocks. Otoliths are the preferred structure for age determination, often performed by counting the number of completed 001 zones. Uncertainty regarding the identification of the first translucent zone and the timing of translucent zone formation can cause skewed age determination even in species with otherwise clearly defined annuli. This study addresses an inter‐institutional discrepancy in age readings of the East Greenland Atlantic cod Gadus morhua L. By analysing the opacity of the otolith margin of cod sampled monthly from February to December, it was shown that translucent zones are formed primarily during March–May. Size distributions of the dorsal–ventral and proximal–distal widths of the first three translucent zones are provided as guide to their identification. This study provides new knowledge that resolves the discrepancy in age reading of East Greenland Atlantic cod.

Projected impacts of warming seas on commercially fished species at a biogeographic boundary of the European continental shelf

Abstract Projecting the future effects of climate change on marine fished populations can help prepare the fishing industry and management systems for resulting ecological, social and economic changes. Generating projections using multiple climate scenarios can provide valuable insights for fisheries stakeholders regarding uncertainty arising from future climate data. Using a range of climate projections based on the Intergovernmental Panel on Climate Change A1B, RCP4.5 and RCP8.5 climate scenarios, we modelled abundance of eight commercially important bottom dwelling fish species across the Celtic Sea, English Channel and southern North Sea through the 21st century. This region spans a faunal boundary between cooler northern waters and warmer southern waters, where mean sea surface temperatures are projected to rise by 2 to 4°C by 2098. For each species, Generalized Additive Models were trained on spatially explicit abundance data from six surveys between 2001 and 2010. Annual and seasonal temperatures were key drivers of species abundance patterns. Models were used to project species abundance for each decade through to 2090. Projections suggest important future changes in the availability and catchability of fish species, with projected increases in abundance of red mullet Mullus surmuletus L., Dover sole Solea solea L., John dory Zeus faber L. and lemon sole Microstomus kitt L. and decreases in abundance of Atlantic cod Gadus morhua L., anglerfish Lophius piscatorius L. and megrim Lepidorhombus whiffiagonis L. European plaice Pleuronectes platessa L. appeared less affected by projected temperature changes. Most projected abundance responses were comparable among climate projections, but uncertainty in the rate and magnitude of changes often increased substantially beyond 2040. Synthesis and applications. These results indicate potential risks as well as some opportunities for demersal fisheries under climate change. These changes will challenge current management systems, with implications for decisions on target fishing mortality rates, fishing effort and allowable catches. Increasingly flexible and adaptive approaches that reduce climate impacts on species while also supporting industry adaptation are required.

Anisakid nematode larvae in the liver of Atlantic cod Gadus morhua L. from West Greenland.

Anisakid nematode larvae occur frequently in the liver of Atlantic cod, but merely few infection data from cod in waters around Greenland exist. The present study reports the occurrence of third-stage anisakid larvae in the livers of 200 Atlantic cod caught on fishing grounds along the West coast of Greenland (fjord systems of Maniitsoq) in May, June, August and September 2017. Classical and molecular helminthological techniques were used to identify the nematodes. A total of 200 cod livers were examined, and 194 were infected with third-stage nematode larvae (overall prevalence of infection 97%) with a mean intensity of 10.3 (range between 1 and 44 parasites per fish). Prevalences recorded were 96% for Anisakis simplex (s.l.), 55% for Pseudoterranova decipiens (s.l.) and 8% for Contracaecum osculatum (s.l.). Sequencing the mtDNA cox2 from 8 out of 23 these latter larvae conferred these to C. osculatum sp. B. A clear seasonal variation was observed, with a rise in A. simplex (s.l.) and P. decipiens (s.l.) occurrence in June and August and a decline in September. The study may serve as a baseline for future investigations using the three anisakids as biological indicators in Greenland waters.

SNP genotyping reveals substructuring in weakly differentiated populations of Atlantic cod (Gadus morhua) from diverse environments in the Baltic Sea

Atlantic cod (Gadus morhua) is one of the most important fish species in northern Europe for several reasons including its predator status in marine ecosystems, its historical role in fisheries, its potential in aquaculture and its strong public profile. However, due to over-exploitation in the North Atlantic and changes in the ecosystem, many cod populations have been reduced in size and genetic diversity. Cod populations in the Baltic Proper, Kattegat and North Sea have been analyzed using a species specific single nucleotide polymorphism (SNP) array. Using a subset of 8,706 SNPs, moderate genetic differences were found between subdivisions in three traditionally delineated cod management stocks: Kattegat, western and eastern Baltic. However, an FST measure of population differentiation based on allele frequencies from 588 outlier loci for 2 population groups, one including 5 western and the other 4 eastern Baltic populations, indicated high genetic differentiation. In this paper, differentiation has been demonstrated not only between, but also within western and eastern Baltic cod stocks for the first time, with salinity appearing to be the most important environmental factor influencing the maintenance of cod population divergence between the western and eastern Baltic Sea.

Ringed seal (Pusa hispida) diet on the west coast of Spitsbergen, Svalbard, Norway: during a time of ecosystem change

Global warming is causing Atlantification of water masses and concomitant changes in food webs in the Barents Sea region. To determine whether changes that have been documented at lower trophic levels are impacting the diet of ringed seals (Pusa hispida) gastrointestinal tracts (GITs) from 99 coastal-feeding ringed seals, collected in western Spitsbergen, Svalbard, were analysed via identification of hard-parts. The study animals were shot in spring (n = 30; April–July) or autumn (n = 69; August–October) during four consecutive years (2014–2017). Thirty different prey types were identified, but most seals (55.6%) had consumed between 2 and 4 different types of prey. Polar cod (Boreogadus saida) dominated the diet of the ringed seals in terms of relative biomass (Bi = 60.0%) and frequency of occurrence (FOi = 86.9%), followed by pricklebacks (Stichaeidae; Bi = 23.4%; FOi = 79.8%). Redundancy analysis (RDA) revealed that year was the only significant predictor explaining variance in autumn diet composition (RDA, F3 = 4.96, AIC = − 76.49, p ≤ 0.0050; blubber content and maturity/sex group were not significant). Blue whiting (Micromesistius poutassou) occurred in the diet in small quantities; this Atlantic fish species has not previously been documented in the ringed seals’ diet. Atlantic cod (Gadus morhua) had the highest Bi (9.2%) among Atlantic prey types. However, despite major changes in the last decade in the fish and zooplankton community in western Svalbard, and consumption of a few Atlantic prey types, the ringed seals’ diet in Svalbard continues to be dominated by Arctic prey, especially polar cod.

Acoustic scattering layers reveal a faunal connection across the Fram Strait

Acoustic scattering layers (SL) at various depths are common phenomena in most oceans, but the organisms that make up these layers vary and so does their density, and hence the backscattered energy. During two crossings of the deep Fram Strait between the shelves at Svalbard and Northeast Greenland at latitudes 77°N and 79°N, we registered epipelagic and mesopelagic SL across the entire Fram Strait and quantified their acoustic backscattered energy. In addition, one pelagic trawl haul was made at each crossing together with a CTD cast at the northern crossing. The epipelagic SL was present at 0–200 m depth, whereas the mesopelagic SL was located at 300–500 m depth during day and at shallower depths during night indicating diel vertical migrations. The epipelagic SL consisted of young-of-the-year fishes, mostly redfish Sebastes sp. No trawl hauls were made in the mesopelagic SL, and the identity of organisms is unknown. Few strong echoes from single targets at mesopelagic depths stood out from the rest of the targets and were interpreted as adult Atlantic cod Gadus morhua. This is the first report of scattering layers covering the whole distance of the deep parts of the Fram Strait, and strengthen the assumption about an east–west connection of organisms and young-of-the-year fishes originating from the spawning grounds along the Norwegian coast and the western Barents Sea towards Northeast Greenland.

A decade of monitoring Atlantic cod Gadus morhua spawning aggregations in Massachusetts Bay using passive acoustics

Atlantic cod Gadus morhua populations in the northeast USA have failed to recover since major declines in the 1970s and 1990s. To rebuild these stocks, managers need reliable information on spawning dynamics in order to design and implement control measures; discovering cost-effective and non-invasive monitoring techniques is also favorable. Atlantic cod form dense, site-fidelic spawning aggregations during which they vocalize, permitting acoustic detection of their presence at such times. The objective of this study was to detect spawning activity of Atlantic cod using multiple fixed-station passive acoustic recorders to sample across Massachusetts Bay during the winter spawning period. A generalized linear modeling approach was used to investigate spatio-temporal trends of cod vocalizing over 10 consecutive winter spawning seasons (2007-2016), the longest such timeline of any passive acoustic monitoring of a fish species. The vocal activity of Atlantic cod was associated with diel, lunar, and seasonal cycles, with a higher probability of occurrence at night, during the full moon, and near the end of November. Following 2009 and 2010, there was a general decline in acoustic activity. Furthermore, the northwest corner of Stellwagen Bank was identified as an important spawning location. This project demonstrated the utility of passive acoustic monitoring in determining the presence of an acoustically active fish species, and provides valuable data for informing the management of this commercially, culturally, and ecologically important species.

Incubation Temperature Effect on Qualitative and Quantitative Composition of Abnormalities and Mortality Rate in Embryogenesis of the Barents Sea Capelin Mallotus villosus (Osmeridae)

The development of capelin Mallotus villosus from insemination to total resorption of the yolk is investigated at 5 and 8°C by the assessment of development of each individual (3850 eggs obtained from 77 spawning pairs). In total, ten malformation types are defined in early development. The frequency of their occurrence and embryonic mortality rate during incubation is described. The total malformation number and mortality rate are significantly higher at 8°С than at 5°С. A distinct relationship between early development quality and age or body size of the females is not revealed. In the majority of trials, the progenies of large females are characterized by better quality, but these data are not always significant. The qualitative composition and malformation rate of capelin differ from those observed in Atlantic cod Gadus morhua using the same methods of investigation. In addition, the malformation and mortality rates during early development are substantially lower in capelin than in Atlantic cod.

Settlement processes induce differences in daily growth rates between two co-existing ecotypes of juvenile cod Gadus morhua

Co-existence of sympatric populations of a fish species is a common phenomenon. In the fjords of southern Norway, 2 ecotypes of Atlantic cod Gadus morhua co-exist during their juvenile life stage. Cod of the North Sea ecotype are on average 2 cm larger than the fjord ecotype at the end of their first growing season in October, suggesting either differences in spawning time or differences in growth during the larval and early juvenile phase. We analysed 24352 daily otolith increments from 145 cod sampled in September 2000, 2003, 2008 and 2015 at 2 locations to estimate individual hatching times and daily growth rates. There was no significant difference in hatching time between ecotypes, locations or years. Population-specific models of the effect of year, temperature and fish length on growth rates showed that the North Sea ecotype grew significantly faster than the fjord ecotype, but growth rate differences were small until the juveniles reached ~40 mm and disappeared again in juveniles larger than 110 mm. The size range (40-110 mm) corresponds to the period following settling. This study documents how vital rates (e.g. growth rates) may change rapidly at ontogenetic transition points, leading to different phenotypic trajectories in co-existing ecotypes. It also highlights the importance of settlement as a key life-history transition that may amplify small existing differences in growth rates through positive feedback of size on competitive ability and eventually recruitment.

Atlantic cod Gadus morhua save energy on stone reefs: implications for the attraction versus production debate in relation to reefs

Reefs are structurally complex habitats that are degraded in numerous coastal areas. Structural complexity is often associated with elevated fish abundance, and recent studies have indicated that such structural complexity (e.g. reefs) not only acts as a fish aggregator, but also increases fish production. The objective of this study was to advance this knowledge by investigating if an underlying mechanism of the observed productivity is related to reduced metabolic rates (proxy for energy use) of fish in reef habitats. Using juvenile Atlantic cod Gadus morhua, we tested the hypothesis that fish energy use differs between fish occupying stone reef and sand bottom habitats. Metabolic rate (MO2) was estimated using intermittent flow respirometry in simulated stone reef and sand bottom habitats over 24 h. Results revealed that G. morhua in the stone reef habitat exhibited significantly reduced accumulated MO2 compared to G. morhua in the sand bottom habitat. Likewise, there was a tendency for lower mean standard metabolic rates of the fish in stone reefs, although this pattern was not statistically significant. There are many mechanisms that may underpin elevated productivity in structurally complex habitats such as reefs, including better access to shelter and increased food availability. Our study adds to these mechanisms by showing that G. morhua save energy when occupying stone reefs as compared to sandy bottoms, energy which may be allocated to somatic and gonadal growth.

Transcriptome profiling reveals exposure to predicted end-of-century ocean acidification as a stealth stressor for Atlantic cod larvae

Ocean acidification (OA), a direct consequence of increasing atmospheric CO2 concentration dissolving in ocean waters, is impacting many fish species. Little is known about the molecular mechanisms underlying the observed physiological impacts in fish. We used RNAseq to characterize the transcriptome of 3 different larval stages of Atlantic cod (Gadus morhua) exposed to simulated OA at levels (1179 µatm CO2) representing end-of-century predictions compared to controls (503 µatm CO2), which were shown to induce tissue damage and elevated mortality in G. morhua. Only few genes were differentially expressed in 6 and 13 days-post-hatching (dph) (3 and 16 genes, respectively), during a period when maximal mortality as a response to elevated pCO2 occurred. At 36 dph, 1413 genes were differentially expressed, most likely caused by developmental asynchrony between the treatment groups, with individuals under OA growing faster. A target gene analysis revealed only few genes of the universal and well-defined cellular stress response to be differentially expressed. We thus suggest that predicted ocean acidification levels constitute a “stealth stress” for early Atlantic cod larvae, with a rapid breakdown of cellular homeostasis leading to organismal death that was missed even with an 8-fold replication implemented in this study.

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea.

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measured (total length, LT = 161-890 mm and weighed (mass, M = 45-6900 g) both before freezing and after defrosting. The cod were found to decrease significantly in both LT and M following death and frozen storage. There was an average (±SD) change in LT of -2.91% (±0.05%) following freezing, independent of starting LT . Total M changed by -2.65% (±0.14%), independent of starting mass. Shrinkage of LT and M did not differ significantly between 1 and 4 months frozen storage, though LT shrinkage was significantly greater after 1 or 4 months in the freezer compared with after 5 days. There was significant variation in LT and M shrinkage between regions of capture. A significant negative relationship between condition of cod and LT or M change was also observed. Equations to back-calculate fresh LT and M from thawed LT , M and standard length (LS ), gutted LT , gutted LS and gutted M are provided.

Strong selection pressures maintain divergence on genomic islands in Atlantic cod (Gadus morhua L.) populations

BackgroundTwo distinct populations have been extensively studied in Atlantic cod (Gadus morhua L.): the Northeast Arctic cod (NEAC) population and the coastal cod (CC) population. The objectives of the current study were to identify genomic islands of divergence and to propose an approach to quantify the strength of selection pressures using whole-genome single nucleotide polymorphism (SNP) data. After applying filtering criteria, information on 93 animals (9 CC individuals, 50 NEAC animals and 34 CC × NEAC crossbred individuals) and 3,123,434 autosomal SNPs were used.ResultsFour genomic islands of divergence were identified on chromosomes 1, 2, 7 and 12, which were mapped accurately based on SNP data and which extended in size from 11 to 18 Mb. These regions differed considerably between the two populations although the differences in the rest of the genome were small due to considerable gene flow between the populations. The estimates of selection pressures showed that natural selection was substantially more important than genetic drift in shaping these genomic islands. Our data confirmed results from earlier publications that suggested that genomic islands are due to chromosomal rearrangements that are under strong selection and reduce recombination between rearranged and non-rearranged segments.ConclusionsOur findings further support the hypothesis that selection and reduced recombination in genomic islands may promote speciation between these two populations although their habitats overlap considerably and migrations occur between them.