Entire Spawning Research Articles

Comparative characteristics for producers of pink salmon <i>Oncorhynchus gorbuscha</i>in the basin of Lake Sopochnoe

Pink salmon spawners in the channel connecting Lake Sopochnoe (Iturup Island) with the sea are investigated. The samples were collected in July-August, 2016. The males dominated significantly during entire spawning run. The fish became more and more ready for spawning with approaching to the end of the run. The average length and weight of females were 50.0 ± 0.3 cm and 1.53 ± 0.03 kg, the average length and weight of males — 50.2 ± 0.2 cm and 1.56 ± 0.02 kg; the fish caught in the channel were significantly larger than the pink salmon spawners caught in the adjacent Prostor Bay at the same time (on average 48.5 ± 0.1 cm, 1.32 ± 0.01 kg and 49.0 ± 0.2 cm, 1.40 ± 0.02 kg for females and males, respectively).

Spatial distribution and interannual variability of catches for pink salmon of artificial origin in the waters of eastern coast of Sakhalin Island in modern times

Mixed herds of pink salmon including the fish of both artificial and natural origin are exploited by fishery in many areas of Sakhalin Island. To separate the fish of different origin, large-scale tagging of the juveniles otoliths is conducted at salmon hatcheries since 2008. The hatcheries contribution to pink salmon catches in certain areas of East Sakhalin (from the Langeri River to Cape Crillon) is assessed for 2011–2017 on the base of previously published and new data on occurrence of the fish with marked otoliths. The specimens were sampled from commercial catches of 5–13 fixed nets and from research catches in 5–9 rivers throughout entire spawning season. In total, 40,992 otoliths of pink salmon were collected. The otoliths were prepared for microstructure analysis using standard methods of processing. All marked otoliths had the tags of hatcheries located on the eastern coast of Sakhalin. The fish of artificial origin were caught in all examined sites between Cape Soymonov and Cape Crillon every year, but in some years their distribution was wider, up to the northern limit of the investigated area (Langeri River). The fish of artificial origin migrated to all rivers within this area. The total annual landing of the pink salmon marked at artificial hatcheries in the East Sakhalin waters was estimated as 300–10600 t, by years. The rest of the fish of artificial origin (0.4–2.3 . 106 ind.) entered to the rivers and mostly reached the spawning grounds. Long-term dynamics of landing was compared for the pink salmon of artificial and natural origin and their independence was concluded. Distribution of pink salmon originated from certain artificial hatcheries by sites of catch was rather constant that allowed to extrapolate the results for marked fish to all released pink salmon. The highest contribution of pink salmon of artificial origin was noted for catches in the Aniva Bay (23.9 %, on average), it was lower for catches at the southeastern coast of Sakhalin (13.2 %), and more lower for catches in the Patience Bay (7.6 %). Total number of pink adults of artificial origin caught on a particular stretch of coast did not depend directly on the number of pink juveniles released from a local hatchery, but was determined mainly by number of fish migrated there from other hatcheries. The landing of pink salmon originated from artificial hatcheries directly in the rivers where these hatcheries were located and at their mouths was < 10 % of the total catch of pink salmon of artificial origin. Schemes of quantitative distribution of pink salmon with marked otoliths in the waters of East Sakhalin are presented.

Influence of Abiotic Environmental Factors on the Abundance of Cod Eggs and Cod Larvae in the Southeastern Baltic Sea in 2016

The characteristics of the abiotic environmental parameters obtained during in situ observations and significant for the survival of cod eggs and larvae (water salinity, water temperature, and oxygen content) are presented for the Baltic Sea for 2016. Cod eggs were present in ichthyoplankton during the entire spawning season (March–August). In July and August 2016, the abundance of cod larvae exceeded the long-term average value for the Gdansk Deep. The thickness of the reproductive layer, suitable for cod reproduction, calculated from vertical localization of 11‰ isohaline and 2 mL/L isooxygene, was sufficient for cod egg survival. Layers with acute vertical density gradients were noted in the upper part of the halocline in March–August and also in the surface water layer in the summer period. It is assumed that the lower boundary of the pycnocline in the surface water layer could serve as a food-rich zone of cod larvae concentration.

Changes in thiamine concentrations, fatty acid composition, and some other lipid-related biochemical indices in Baltic Sea Atlantic salmon (Salmo salar) during the spawning run and pre-spawning fasting

Salmonines in the Baltic Sea and North American lakes suffer from thiamine (vitamin B1) deficiency, which is connected to an abundant lipid-rich diet containing substantial amounts of polyunsaturated fatty acids (PUFAs). In the Baltic region, this is known as the M74 syndrome. It affects both adult salmon (Salmo salar) and especially their offspring, impairing recruitment. However, very little is known about the thiamine and lipid metabolism of salmon during feeding and spawning migrations in the Baltic Sea. In this study, salmon females were sampled along the spawning run from the southern Baltic Proper in four locations at sea and finally at spawning in a river at the Bothnian Bay in a year with insignificant M74 mortality. Changes in concentrations of thiamine and its components in muscle, ovaries, and the liver and other biochemical indices potentially relating to lipid and fatty acid metabolism were investigated. The results provide further evidence of the role of peroxidation of PUFAs in eliciting thiamine deficiency in salmon: During the entire spawning run, the muscle total lipid content decreased by 50%, palmitic acid (16:0) by 62%, and docosahexaenoic acid (DHA, 22:6n-3) by 45%. The concentration of total thiamine decreased significantly until the spawning in the liver and ovaries, 66 and 70% respectively. In the muscle, the proportion of thiamine pyrophosphate of total thiamine increased with the use of muscular lipid stores. There was no trend in the concentration of total carotenoids during the spawning run. The doubling of the concentration of hepatic malondialdehyde indicated peroxidation of PUFAs, and the mobilisation of body lipids suppressed the activity of glucose-6-phosphate dehydrogenase, as consumed dietary lipids would also have done.

Spotted wolffish (Anarhichas minor) sperm cryopreservation in 5-mL cryovials

In spotted wolffish Anarhichas minor aquaculture, cryopreservation is used to secure sperm availability throughout the entire spawning season. Under current protocols, sperm is cryopreserved in 0.5-mL straws. This implies thawing a considerable number of straws for insemination with cryopreserved sperm. In this work, we scale up the spotted wolffish sperm cryopreservation procedure through the development of a protocol for sperm cryopreservation in 5-mL cryovials. Different freezing (distances from the liquid nitrogen surface) and thawing rates were tested. The best results were obtained with cryovials frozen at a distance of 1.5 cm from the liquid nitrogen surface and thawed either at 15 or 10 °C for 4 and 6 min, respectively. Under these conditions, similar percentage of motile cells, sperm velocity and percentage of viable cells were obtained in comparison with the sperm cryopreserved in the traditional 0.5-mL straws. This protocol will facilitate the process of insemination with cryopreserved sperm in the spotted wolffish hatcheries.

Distribution, Spawning and the Possibility of Fishery of Introduced Pink Salmon (Oncorhynchusgorbusha Walbaum) in Rivers of Murmansk Oblast

Migration patterns of pink salmon, its distribution along the spawning grounds, spawning success, and the potential impact on the reproduction of the native species Atlantic salmon (Salmo salar L.) have been studied in the largest salmon rivers of the White Sea basin on the Kola Peninsula, the Ponoy, Varzuga, and Umba rivers. It is shown that the distribution of pink salmon is not uniform in the rivers with different hydrological characteristics. When choosing spawning grounds, pink salmon prefers shallow riffles with moderate current and fine-fraction bottom sediments. Checking of the spawning redds indicates a poor spawning success of pink salmon. To date, there is no convincing proof of the influence of the invader on the ecosystems of the rivers. For the efficient use of the resource, unrestricted removal of pink salmon by all types of fishing has been proposed, including total catch using fish-counting barriers (FCBs) in the Umba and Varzuga rivers during the entire spawning run.

The relationship between channel catfish female body weight and relative fecundity and fry production when induced to ovulate with carp pituitary extract and fertilized with blue catfish sperm

ABSTRACTLittle is known regarding the correlation between channel catfish female body weight and relative fecundity and fry production when the females are induced to ovulate with carp pituitary extract and fertilized with blue catfish sperm. A total of 145 female channel catfish were used in the early, middle, and late spawning season. Female body weight of channel catfish had negative correlations with relative fecundity (r = −0.33 and −0.25, P < .05; r = −0.23, P > .05) and before the late spawning season with fry/kg (r = −0.21, P > .05; r = −0.30, P < .05). Eggs/kg female body weight and fry/kg female body weight followed a parallel pattern throughout the season, illustrating the logical relationship between relative fecundity and fry output. The size of females used is a consideration to maximize hybrid catfish embryo production during the early and middle spawning season but not late in the spawning season. However, these correlations are relatively small even though significant; thus a dramatic change in fry production would not be predicted. Since reproductive relationships can change during the spawning season, conclusions and recommendations could be flawed if data are not collected periodically during an entire spawning season.

Validation of a methodology to determine female white sturgeon (Acipenser transmontanus Richardson, 1836) habitat use within a riverscape during the spawning season

Summary The objective was to use hydrologic and bathymetric models for the riverscape of the Kootenai River, Idaho, USA and integrate these data with radio telemetry locations of two female white Sturgeon, Acipenser transmontanus Richardson, 1836, during the spawning season. The detailed analysis of habitat features and models provided an enhanced understanding of the habitat preferences of spawning White Sturgeon, which could be used to protect or enhance the habitat. Hydrologic models will also provide a basis to incorporate a larger sample of fish in the future. The White Sturgeon habitats were studied during the 2002 and 2003 spawning seasons. Depth sensitive radio transmitters, geographic positioning systems, single-beam and multibeam echo sounders and multidimensional surface water modeling were used to identify similarities and dissimilarities in the habitat selection of the two monitored female White Sturgeon. The movements of the two fish were not randomly distributed; both fish spent a significant amount of time swimming near the river bottom and thalweg or resting near the river bottom (within 20% of the lower water column [6–9 m]). Mahalanobis distances quantitatively described the similarities between select habitat features of depth, river velocity and riverbed slope at the studied locations and compared the entire spawning reach. The fish occupied sites previously identified as spawning locations. Both fish selected similar depth, median velocity (0.3 m s−1) and bank preferences, avoiding the shallow water on the inside of meander bends and tending to stay in the deeper water on the outside of meanders. They exhibited a tendency to occupy intermediate velocities in parts of the river where the ratio of the velocity at the fish to the maximum velocity in the channel cross-section was >0.25 and <0.75. Some of the differences in habitat selection were likely due to the larger river discharge in 2002 compared to 2003, the length of time each fish was monitored, and the length of the spawning reach occupied by each fish. Some of the differences could, however, have been due to individual preferences. A larger sample size will greatly enhance studies.

Impact of Ichthyophonus infection on spawning success of Yukon River Chinook salmon Oncorhynchus tshawytscha

We examined the impacts of Ichthyophonus infection on spawning success of Yukon River Chinook salmon Oncorhynchus tshawytscha at spawning grounds of the Chena and Salcha Rivers, Alaska, USA. During the period 2005 to 2006, 1281 salmon carcasses (628 male, 652 female) were collected throughout the spawning season and from the entire spawning reaches of the Chena and Salcha Rivers. For each fish, infection status was determined by culture method and visual inspection of lesions of heart tissue as uninfected (culture negative), infected without lesions (culture positive with no visible lesions), and infected with lesions (culture positive with visible lesions), and spawning status was determined by visually inspecting the percentage of gametes remaining as full-spawned (<10%), partial-spawned (10-50%), and unspawned (>50%). Among the 3 groups, the proportion of full-spawned (i.e. spawning success) females was lower for those infected without lesions (69%) than those uninfected (87%) and infected with lesions (86%), but this did not apply to males (uninfected 42%, infected without lesions 38%, infected with lesions 41%). At the population level, the combined (infected and uninfected) proportion of female spawning success was 86%, compared to 87% when all females were assumed uninfected. These data suggest that while Ichthyophonus infection slightly reduces spawning success of infected females, its impact on the spawning population as a whole appears minimal.

Factors Regulating Early Life History Dispersal of Atlantic Cod (Gadus morhua) from Coastal Newfoundland

To understand coastal dispersal dynamics of Atlantic cod (Gadus morhua), we examined spatiotemporal egg and larval abundance patterns in coastal Newfoundland. In recent decades, Smith Sound, Trinity Bay has supported the largest known overwintering spawning aggregation of Atlantic cod in the region. We estimated spawning and dispersal characteristics for the Smith Sound-Trinity Bay system by fitting ichthyoplankton abundance data to environmentally-driven, simplified box models. Results show protracted spawning, with sharply increased egg production in early July, and limited dispersal from the Sound. The model for the entire spawning season indicates egg export from Smith Sound is 13%•day−1 with a net mortality of 27%•day–1. Eggs and larvae are consistently found in western Trinity Bay with little advection from the system. These patterns mirror particle tracking models that suggest residence times of 10–20 days, and circulation models indicating local gyres in Trinity Bay that act in concert with upwelling dynamics to retain eggs and larvae. Our results are among the first quantitative dispersal estimates from Smith Sound, linking this spawning stock to the adjacent coastal waters. These results illustrate the biophysical interplay regulating dispersal and connectivity originating from inshore spawning of coastal northwest Atlantic.

Length-weight relationships, condition factor, gonadosomatic index-based size at first sexual maturity, spawning season and fecundity of Aspidoparia morar (Cyprinidae) in the Jamuna River (Brahmaputra River distributary), northern Bangladesh

Summary The present study describes the length-weight relationships (LWRs), length-length relationships (LLRs), Fulton's condition factor (KF), size at first sexual maturity, spawning season, sex ratio and fecundity of the Morari Aspidoparia morar (Hamilton, 1822) (Cyprinidae). Sampling was done using traditional fishing gear jhaki jal (cast net) from July 2010 to June 2011. Total length (TL), fork length (FL) and standard length (SL) were measured with digital slide calipers. Individual body weight (BW) and gonad weight (GW) were determined to an accuracy of 0.01 g for all specimens. The gonadosomatic index (GSI) was calculated and size at first maturity for males and females estimated using GSI and TL as indicators. Female ≥ size at first maturity was used to determine fecundity. A total of 1200 specimens (males = 552, females = 648) ranging from 4.06–12.84 cm TL and 0.53–16.75 g BW were analyzed. The overall coefficient b for the LWR indicated positive allometric growth (>3.00) in males and isometric growth in females (~ 3.00). ancova (analysis of covariance) revealed significant differences between males and females (P 0.973, P < 0.001). Sizes at first sexual maturity for males and females were 6.0 and 7.0 cm TL, respectively. KF changed little throughout the year and GSI peaked in November to April, indicating the spawning season (GSImax = 15.0 in females, 2.0 in males). Mature females were dominant during the entire spawning season except in April. Mean total fecundity was 6700 ± 3500, ranging from 1860 to 19680. In addition, relative fecundity ranged from 190 to 1200 (mean 560 ± 235) in the Jamuna River. To ensure sustainable management of this species, the protection of mature individuals during the peak spawning season is highly recommended.

Recruitment Variability in North Atlantic Cod and Match-Mismatch Dynamics

BackgroundFisheries exploitation, habitat destruction, and climate are important drivers of variability in recruitment success. Understanding variability in recruitment can reveal mechanisms behind widespread decline in the abundance of key species in marine and terrestrial ecosystems. For fish populations, the match-mismatch theory hypothesizes that successful recruitment is a function of the timing and duration of larval fish abundance and prey availability. However, the underlying mechanisms of match-mismatch dynamics and the factors driving spatial differences between high and low recruitment remain poorly understood.Methodology/Principal FindingsWe used empirical observations of larval fish abundance, a mechanistic individual-based model, and a reanalysis of ocean temperature data from 1960 to 2002 to estimate the survival of larval cod (Gadus morhua). From the model, we quantified how survival rates changed during the warmest and coldest years at four important cod spawning sites in the North Atlantic. The modeled difference in survival probability was not large for any given month between cold or warm years. However, the cumulative effect of higher growth rates and survival through the entire spawning season in warm years was substantial with 308%, 385%, 154%, and 175% increases in survival for Georges Bank, Iceland, North Sea, and Lofoten cod stocks, respectively. We also found that the importance of match-mismatch dynamics generally increased with latitude.Conclusions/SignificanceOur analyses indicate that a key factor for enhancing survival is the duration of the overlap between larval and prey abundance and not the actual timing of the peak abundance. During warm years, the duration of the overlap between larval fish and their prey is prolonged due to an early onset of the spring bloom. This prolonged season enhances cumulative growth and survival, leading to a greater number of large individuals with enhanced potential for survival to recruitment.

Red Snapper Reproductive Biology in the Southern Gulf of Mexico

AbstractSex ratio, size at maturity, reproductive periodicity, ovarian development pattern, and spawning ground data for red snapper Lutjanus campechanus from the southern Gulf of Mexico were analyzed to understand this species' reproductive biology throughout its geographical distribution. Red snapper were sampled in 1999 and 2000 from commercial fishery catches taken on Campeche Bank at depths ranging from 43 to 130 m. Overall sex ratio (male: female) and sex ratios by size‐class did not differ significantly from a 1:1 ratio. First maturity was at a slightly smaller size in males (24.2 cm total length [TL]) than in females (28.3 cm TL), and maximum length percentages at first maturity were 28% for males and 34% for females. The length at which maturity was attained in 50% of females (L50) was 31.4 cm TL. The Campeche Bank red snapper stock had a protracted spawning period from February to November, with a spawning peak occurring in early fall. Vitellogenic oocytes were continually recruited from previtellogenic oocytes during the entire spawning season, with no gap between each oocyte development stage. This means that red snapper ovaries are asynchronous and that the species can be considered a heterochronal (batch) spawner with indeterminate annual fecundity. Females in spawning condition were collected from both the northeastern and northwestern areas of Campeche Bank, near submerged or emergent coral reef structures, and at depths between 48 and 117 m. These results generally agreed with those for red snapper stocks in others regions of the Gulf of Mexico and the Atlantic Ocean off the southeastern United States. However, reproductive periodicity in the Campeche Bank red snapper stock apparently displayed more of an insular pattern of extended spawning in comparison with the typical continental pattern of restricted spawning reported for other regions. The latitudinal differences observed in red snapper spawning seasons could also reflect a latitudinal gradient in the intensity of seasonal environmental constraints.

The Spawning Run of Blueback Herring in the St. Johns River, Florida

AbstractBlueback herring Alosa aestivalis are anadromous, iteroparous, winter spawners in the St. Johns River, Florida, but published data about this population are limited. We collected adults by electrofishing in the upper St. Johns River during the 2002–2005 spawning runs to examine the abundance, distribution, size, sex ratio, reproductive biology, and feeding habits of the population. When possible, data from the present study were compared with those for blueback herring collected with seine nets in the lower St. Johns River during the 1972 and 1973 spawning runs. Blueback herring began upstream migration by January and stayed in the river until at least April. The average fish size was significantly smaller in 2002–2005 than in 1972–1973, but sex ratios were not different from 1:1 in 5 of the 6 years studied. The more recent collections suggest that blueback herring females released multiple batches of eggs during one spawning season (spawning a batch every 3–4 d on average) and that spawning occurred as far upstream as river kilometer 400. Males and females fed actively during the entire spawning run. Overall, the stock is persistent, but its status is unknown and the smaller individual sizes evident today suggest that this population is experiencing higher mortality now than a few decades ago.

When is year-class strength determined in western Baltic herring?

Abstract Oeberst, R., Klenz, B., Gröhsler, T., Dickey-Collas, M., Nash, R. D. M., and Zimmermann, C. 2009. When is year-class strength determined in western Baltic herring? – ICES Journal of Marine Science, 66: 1667–1672. Weekly surveys of larvae in the Strelasund and the Greifswalder Bodden were used to investigate when year-class strength is determined in western Baltic spring-spawning (WBSS) herring. An abundance metric of larvae reaching a length of 20 mm over the entire spawning season was constructed by accounting for increases in daily growth resulting from seasonal increases in temperature (5–20°C). The index was significantly correlated with the acoustic estimates of age-1 herring in the western Baltic Sea (r = 0.88) and with the estimates of year-class strength obtained from stock assessment (r = 0.65). Previous studies of herring elsewhere have reported that year-class strength is determined during the late larval stage, but we show that year-class strength can already be fixed at a larval length of only 20 mm. Although the index obtained may be used in stock assessment and predictions, the intriguing question remains, namely how can the signal of larval productivity from one single spawning component of WBSS herring reflect the year-class dynamics of the entire stock?

Assessing spawning ground hydraulic suitability for Chinese sturgeon ( Acipenser sinensis) from horizontal mean vorticity in Yangtze River

The Chinese sturgeon, Acipenser sinensis, is an anadromous protected species. Its migration pattern has been blocked since the construction of Gezhouba dam and the length of the natural spawning site reduced to less than 7 km. However, the fish eventually established an alternative spawning ground in the narrow downstream reach of Gezhouba dam. In this article, we applied Delft3D-Flow model to simulate the hydraulic suitability of the spawning ground downstream Gezhouba dam. Horizontal mean vorticity was used to assess the hydraulic environment of spawning ground. The flow field state was determined through model simulation and field-measured data used to validate the model. The computational method was improved by calculating absolute horizontal mean vorticity from estimates the literature. The final vorticity was determined from the simulation output and its distribution pattern retrieved. The horizontal mean vorticity range was 0.71–4.61 10 −3 s −1 for the entire spawning grounds, with egg mass field upper limit of 1.0 × 10 −3 s −1. Vorticity strength selection of Chinese sturgeon spawning can enhance our understanding of egg fertilization rate, hence the protection of fertilized eggs. Furthermore, the results of the study would add to existing scientific database for spawning ground hydraulic environment protection, especially in the ecological regulation drive of the Three Gorges Reservoir.

Synchronous Female Spawning and Male Mating Behavior in a Land-Locked Population of Japanese Charr, Salvelinus leucomaenis japonicus

In resource-based promiscuous mating systems, synchronous spawning of females affects competition among males and variation in the reproductive success of males. We documented the mating behavior of Japanese charr (Salvelinus leucomaenis japonicus) through an annual breeding season to examine the relationship between female spawning synchrony and male mating behavior. Females spawned highly synchronously in the population studied, i.e., approximately half the spawning was finished within the first three days of the entire spawning season (11 days). The daily operational sex ratio (OSR) was nearly 1:1 through the spawning period. The number of males around a spawning female was very small (1.21+/-0.49 males per female) over the spawning ground and period, suggesting that a competitive male could effectively chase subordinate males away from a spawning female. A few males attempted to sneak near the oviposition site of females (16%; 9 of 57 breeding groups), while some males adopted sneaking tactics in the initial phase of females' spawning (24%). We did not observe any males to succeed in sneak fertilizations. We conclude that in this Japanese charr population, the synchronous spawning of females was related to the unbiased daily OSR, male aggregation around females, and consequently whether and how efficiently males engaged in sneak mating behavior.

Effect of Photoperiod Manipulation on Broodstock Spawning, Fertilization Success, and Egg Developmental Abnormalities in Atlantic Cod, Gadus morhua

Abstract Spawning activity in two captive Atlantic cod broodstock groups previously captured from the wild stock fishery was monitored over three spawning seasons. Both groups spawned under ambient photoperiod (PP) in the first year, after which photomanipulation was applied to compress the PP cycle of one broodstock group, while the second group was maintained on natural PP. Increased total egg volumes, number of egg batches, total egg production, and duration of the spawning season were observed in both ambient PP and advanced PP groups in the second and third spawning seasons. Compared to ambient controls, the photoadvanced group commenced spawning earlier, had a longer spawning season, produced a lower total volume of eggs over the entire spawning season, and had lower mean daily batch volumes of eggs, and the eggs were of smaller mean diameter. Fertilization success was &gt;90% in all years, increased in both groups in the first year after photomanipulation but declined to prior levels in the following year. No consistent difference in fertilization success was observed between advanced PP and ambient PP groups. Rates of cell development abnormalities were low (&lt;10%) in both groups over the three spawning seasons. Abnormalities of cell symmetry and clarity increased in both ambient PP and advanced PP groups over the three spawning seasons. However, no consistent differences in rate of cell abnormalities were observed between advanced PP and ambient PP groups within years. We conclude that PP advancement is an effective technique to compress the time between successive spawning periods in Atlantic cod and does not negatively affect fertilization success or rates of cell developmental abnormalities. However, it has a negative effect on both volume and size of eggs produced.

Effects of population size/age structure, condition and temporal dynamics of spawning on reproductive output in Atlantic cod ( Gadus morhua)

In this study, we model daily reproductive output over an entire spawning season for a range of simulated age/size-structured populations of Atlantic cod ( Gadus morhua), which are created under contrasting stock–recruitment and fishing pressure scenarios. An individual-based modelling approach is used to link empirical relationships with flexible reproductive parameters to quantify and qualify the effects that individual female condition and egg quality can have on stock reproductive potential (SRP) and the temporal distribution of reproductive output. The results of this study indicate that, even with a set of steady state populations that have equal spawning stock biomass (SSB), the differences in population structure brought about by a difference of fishing mortality ( F) from 0 to 1.0 will substantially reduce the SRP. Compared to a non-fished population the decline in SRP will range from 48 to 74%, depending upon respective assumptions of egg size on viability. The truncation of population age/size structure due to fishing mortality will also lead to a 4 weeks shorter spawning season and a 2 weeks shift in the date of peak spawning. The effect of low condition of individuals can lead to almost total reproductive failure for the population and the effect of increased condition is dependent upon the population structure in question. Results as such support the view that predictions of recruitment can no longer be based solely as a function of SSB. More careful analysis of the population structure as well as annual dynamics of condition factors and exploration of functional relationships between female characteristics and egg quality are needed to assess and accurately predict survival and the probability of recruitment levels.

Male reproductive competition in spawning aggregations of cod (Gadus morhua, L.)

Reproductive competition may lead to a large skew in reproductive success among individuals. Very few studies have analysed the paternity contribution of individual males in spawning aggregations of fish species with huge census population sizes. We quantified the variance in male reproductive success in spawning aggregations of cod under experimental conditions over an entire spawning season. Male reproductive success was estimated by microsatellite-based parentage analysis of offspring produced in six separate groups of spawning cod. In total, 1340 offspring and 102 spawnings distributed across a spawning season were analysed. Our results show that multiple males contributed sperm to most spawnings but that paternity frequencies were highly skewed among males, with larger males on average siring higher proportions of offspring. It was further indicated that male reproductive success was dependent on the magnitude of the size difference between a female and a male. We discuss our results in relation to the cod mating system. Finally, we suggest that the highly skewed distribution of paternity success observed in cod may be a factor contributing to the low effective population size/census population size ratios observed in many marine organisms.