Effects of commercial fishing on the population structure of spawning southern calamary (Sepioteuthis australis)

This study explored population dynamics of Sepioteuthis australis on a fine temporal scale before, during, and after a 3-month commercial fishing closure on the summer inshore spawning grounds of Great Oyster Bay, Tasmania, Australia. An abrupt change in male size (mantle length) and population sex ratio after the re-opening of the commercial fishery suggests that fishing alters the population structure on the spawning beds from a ‘natural’ structure which is highly biased towards males, to a more even ratio of males to females. Although jigs are taking a representative sample of the squid population using the spawning beds at any one time, the fishery is apparently still effectively selective for males, potentially as a function of differential spawning movements of the two sexes. Increased fishing pressure over the past 5 years had been correlated with a change towards a highly male-biased sex ratio on the spawning beds; however the current study suggests that increased fishing pressure in recent years may not have reduced the proportion of females. Instead, the inter-annual change in sex ratio may reflect changes in the degree of protection from fishing, with progressively longer closures allowing time for more males to accumulate on the beds and therefore, for the proportion of males to increase (as opposed to a decrease of females). As fishing is selective for males, fishing throughout the spawning season could potentially modify the process of sexual selection and the mating behaviors of the individuals within the spawning population, highlighting the need for closures over this crucial period. Additionally, examinations and comparisons of squid population structure need to be interpreted in light of fishing pressure and broader movement patterns.     

To migrate, or not to migrate: partial diel horizontal migration of fish in a temperate freshwater reservoir

The diel horizontal migration (DHM) of fish between the inshore and offshore zones of the ?ímov Reservoir (Czech Republic, deep, stratified, meso-eutrophic) was investigated by a combination of horizontal and vertical hydroacoustic surveys at 3-h intervals over 48 h and day/night purse seining in August 2007. An overwhelming majority of fish were aggregated within the epilimnetic layer. Considering only the horizontal surveys, cyclic diel fish movements between inshore and offshore habitats were apparent, while the total fish biomass remained constant between day and night. A higher fish biomass was detected in the offshore zone during daytime by both hydroacoustics and purse seining. In contrast, a higher fish biomass was recorded at night in the inshore zone. Bream Abramis brama, roach Rutilus rutilus, and perch Perca fluviatilis dominated the daytime offshore fish assemblage whereas bleak Alburnus alburnus prevailed at night. Bream and roach decreased in abundance at night while perch completely disappeared from the offshore habitat. The diel differences in size distributions and direct catches suggested the population-wide horizontal offshore migration of bleak and inshore migration of all perch during dusk. On the other hand, partial inshore migration of bream and roach adults was observed during dusk (52 and 80%, respectively). The different proportions of offshore residents among species and size classes suggested that differences in size, and, therefore, predation vulnerability, contributed to the observed migration patterns.     

VMS-based fishing effort and population demographics for the European squid (Loligo vulgaris) off the Portuguese coast

This study presents a 1-year synopsis of the trawl fishery for the European squid Loligo vulgaris in Portuguese waters, integrating length-structured landings with corresponding geo-referenced fishing activities. From vessel monitoring system (VMS) data, landings and biological sampling, a “status-report” was obtained for 2003. Fishing pressure was found to be most intense in inshore areas of the northwest and the south coasts. Population size structure, classified into three categories, was not uniform throughout the fishing areas. Larger squid are found offshore in the northwest (International Council for the Exploration of the Sea—ICES rectangle 10E0) and in the south (rectangles 2E1 and 3E1), whereas all inshore western coast rectangles showed larger proportions of small squid relative to the southern rectangles. The analysis carried out in this study provides an insight into the possible impact of the fishing intensity pattern on the population structure in various zones. The results demonstrate the benefits of combining geo-referenced fisheries information with landings and size data, to produce explicit spatio-temporal information that can contribute to integrated planning and management for the sustainable exploitation of this resource.     

Habitat specialization in tropical continental shelf demersal fish assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1-10 m depth), down the fore reef slope to the reef base (10-30 m depth) then across the adjacent continental shelf (30-110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.     

The Transparent Goby, Aphia MinutaReview of Biology and Fisheries of a Paedomorphic European Fish

The transparent goby, Aphia minuta, is one of the few pelagic species of the family Gobiidae. Its geographic range covers the eastern Atlantic Ocean, from Gibraltar to Norway and the Mediterranean basin including the Azov and Black seas. However, it does not occur along the north African coasts. The species is characterized by a unique larval morphology and precocious sexual maturation, i.e. a type of heterochrony known as progenesis. The transparent goby is an annual species that lives less than 1 year, with a maximum size of less than 60 mm. The breeding season is quite long and spawning takes place at least twice during its short lifespan. Just after spawning, all breeders quickly die probably as a consequence of the degeneration of the intestinal epithelium by apoptosis, marking the end of the entire cohort. Spending most of its life in the water column, it is a planktotrophic feeder, relying mainly on small copepods. In turn the transparent goby is preyed on by several species of fish and larger cephalopods. During ontogenesis, t hree phases characterize the species: a “pelagic phase”, composed of larval stages hatched from demersal eggs that inhabit shallow coastal waters; an “aggregated phase”, composed of juveniles that gather in schools in shallow waters during winter; finally, a “demersal phase”, composed of adults that in spring migrate offshore with a more dispersed distribution in proximity of the bottom. Despite its small size, this species is largely exploited by local small-scale fisheries developed mainly in the western and central Mediterranean. The fishing fleets, harboured in several sites in Spain and Italy, are generally composed of small vessels. Most of them use very selective gear such as purse-seine nets to catch schools of the transparent goby during winter. For these artisanal fisheries, the transparent goby represents, although seasonally, a very important source of income, yielding locally up to more than 100 tons per fishing season and 75 kg/vessel/day. This review critically examines the published literature on the biology and fisheries of the transparent goby, aiming to provide useful tools for appropriate management and for a sustainable exploitation of this important resource.     

Reproductive ecology of Japanese anchovy off the Pacific coast of eastern Honshu, Japan

Spawning frequency of offshore migrant populations of Japanese anchovy Engraulis japonicus ranged from 0·34 to 0·90, and was higher than that of inshore Japanese anchovy. In addition, spawning frequency of offshore populations varied in response to the sex ratio. They spawned at water temperatures of 5.0, 8.6 and 12.6° C, indicating that the minimum critical water temperature of offshore Japanese anchovy was considerably lower than that of inshore populations. A significant positive relationship was observed between water temperature and relative batch fecundity of offshore Japanese anchovy. Furthermore, water temperature was negatively related to egg size, suggesting a trade-o. between relative batch fecundity and egg size for offshore fish. At the same water temperature, relative batch fecundity of offshore populations was higher than that of inshore ones. The comparison of spawning frequency and relative batch fecundity suggests that offshore Japanese anchovy represent a reproductive ecology by which they spawn more eggs than inshore Japanese anchovy within a certain time period.     

The novel use of pop‐off satellite tags (PSATs) to investigate the migratory behaviour of European sea bass Dicentrarchus labrax

A total of 12 adult European sea bass Dicentrarchus labrax were tagged with pop‐off satellite archival tags (PSAT) in Irish coastal waters and in offshore waters in the north‐east Celtic Sea between 2015 and 2016. Archived data were successfully recovered from five of the 12 tags deployed, three from fish released in inshore Irish waters and two from fish released offshore in the eastern Celtic Sea. All three fish tagged in inshore waters were found to undertake migrations into the open ocean coinciding with the spawning period. These fish also exhibited fidelity to inshore sites post‐migration, returning to the same general location (within c. 73 km, which is roughly the predicted mean accuracy of the method) of their original release site. Although the number of tracks obtained here was limited, some degree of aggregation between inshore and offshore tagged fish in the eastern Celtic Sea was noted during the expected spawning period suggesting PSATs can provide new information on specific spawning locations of European sea bass.     

Genetic Structure of Capelin (Mallotus villosus) in the Northwest Atlantic Ocean

Capelin (Mallotus villosus) is a commercially exploited, key forage-fish species found in the boreal waters of the North Pacific and North Atlantic Oceans. We examined the population structure of capelin throughout their range in the Canadian northwest Atlantic Ocean using genetic-based methods. Capelin collected at ten beach and five demersal spawning locations over the period 2002 through 2008 (N = 3,433 fish) were genotyped using six polymorphic microsatellite loci. Temporally distinct samples were identified at three beach spawning locations: Chance Cove, Little Lawn and Straitsview, Newfoundland. Four capelin stocks are assumed for fisheries management in the northwest Atlantic Ocean based on meristics, morphometrics, tag returns, and seasonal distribution patterns. Our results suggested groupings that were somewhat different than the assumed structure, and indicate at least seven genetically defined populations arising from two ancestral populations. The spatial mosaic of capelin from each of the two basal cluster groups explains much of the observed geographic variability amongst neighbouring samples. The genetic-defined populations were resolved at Jost’s D _est ≥ 0.01 and were composed of fish collected 1) in the Gulf of St. Lawrence, 2) along the south and east coasts of Newfoundland, 3) along coastal northern Newfoundland and southern Labrador, 4) along coastal northern Labrador, 5) near the Saguenay River, and at two nearshore demersal spawning sites, 6) one at Grebes Nest off Bellevue Beach on the east coast of Newfoundland, and 7) one off the coast of Labrador at Domino Run. Moreover, the offshore demersal spawners on the Scotian Shelf and Southeast Shoal appeared to be related to the inshore demersal spawners at Grebes Nest and in Domino Run and to beach spawners from the Gulf of St. Lawrence.     

Dining hall at sea: feeding migrations of nektonic predators to the eastern Patagonian Shelf

Seasonal changes in relative abundance and biomass of nektonic predators were analysed on the eastern Patagonian Shelf and continental slope; one of the most productive large marine ecosystems of the southern hemisphere. Several migratory types were revealed for species belonging to either temperate or sub-Antarctic faunas. Despite high productivity, only a few large nektonic predators spend their entire life cycle on the eastern Patagonian Shelf and use only a small proportion of the meso-nektonic resource. Most of the resource is exploited by non-resident nektonic migrants, which move to the area from distant spawning grounds. Pelagic and demersal sharks and skates, the squid Illex argentinus, tunas and gadoids migrate to the eastern part of the Patagonian Shelf to feed at different times of the year; arriving in seasonal waves according to their life cycle and spawning seasonality. Some deepwater fishes and squid migrate onto the shelf as juveniles to harvest the resource, and then return to deepwater habitat as adults. It is hypothesized that the large biomass of meso-planktonic and meso-nektonic consumers prevents most higher-trophic level predators from establishing spawning populations in this area, as their larvae and fry would be overwhelmed by predation. Instead, the higher-trophic level predators establish spawning and nursery grounds elsewhere and arrive to feed on the meso-planktonic and meso-nektonic resources after they have outgrown their own stages of predation vulnerability.     

The Role of Carrion Supply in the Abundance of Deep-Water Fish off California

Few time series of deep-sea systems exist from which the factors affecting abyssal fish populations can be evaluated. Previous analysis showed an increase in grenadier abundance, in the eastern North Pacific, which lagged epibenthic megafaunal abundance, mostly echinoderms, by 9–20 months. Subsequent diet studies suggested that carrion is the grenadier''s most important food. Our goal was to evaluate if changes in carrion supply might drive the temporal changes in grenadier abundance. We analyzed a unique 17 year time series of abyssal grenadier abundance and size, collected at Station M (4100 m, 220 km offshore of Pt. Conception, California), and reaffirmed the increase in abundance and also showed an increase in mean size resulting in a ∼6 fold change in grenadier biomass. We compared this data with abundance estimates for surface living nekton (pacific hake and jack mackerel) eaten by the grenadiers as carrion. A significant positive correlation between Pacific hake (but not jack mackerel) and grenadiers was found. Hake seasonally migrate to the waters offshore of California to spawn. They are the most abundant nekton species in the region and the target of the largest commercial fishery off the west coast. The correlation to grenadier abundance was strongest when using hake abundance metrics from the area within 100 nmi of Station M. No significant correlation between grenadier abundance and hake biomass for the entire California current region was found. Given the results and grenadier longevity, migration is likely responsible for the results and the location of hake spawning probably is more important than the size of the spawning stock in understanding the dynamics of abyssal grenadier populations. Our results suggest that some abyssal fishes'' population dynamics are controlled by the flux of large particles of carrion. Climate and fishing pressures affecting epipelagic fish stocks could readily modulate deep-sea fish dynamics.     

Disintegration of a large concentration of loliginid squid as a response to predation

During hydroacoustic observations in November 2002, a large concentration of chokka squid (Loligo reynaudi) was monitored for three days during fairly constant wind speed and direction, that marginally improved during the period of observations. During this period, the concentration (estimated biomass 48 t) disintegrated into small, separate aggregations. Most often, such dispersal is weather-related, but obviously not in this case. Instead, a bottom trawl made adjacent to the concentration, as well as underwater camera observations revealed an unusually large number of predators, mostly bronze whaler sharks (Carcharhinus brachyurus). Most whaler sharks caught in the trawl had chokka in their stomachs. Therefore, one explanation for the break-up of this squid concentration was the unusual predator activity. Video observations revealed, that these attacks occurred on the bottom where squid spawn; while none were observed in the water column where squid pair, mate and swim in a circular motion preparing for descent to the egg bed. The disintegration of the whole concentration suggested that disrupted spawning affects the upper part of the typical mushroom-shaped structure as well, and squid subsequently disperse and/or move away as a result of predation by whaler sharks.     

Intercalibration of hydroacoustic and mark–recapture methods for assessing the spawning population size of a threatened fish species

Hydroacoustic counting and a three-year mark–recapture study with passive integrated transponders (PIT tags) were used to estimate the size of a spawning population of nase Chondrostoma nasus , a threatened potamodromous cyprinid that undertakes annual spawning migrations into a tributary of the Danube River. In 2005, the estimates of the size of the spawning population from the hydroacoustic counts ( N = 2234, 95% CL 1929–2538) and from the Jolly–Seber model ( N = 1198, 95% CL 461–5842) corresponded well. Estimates from the jackknife-estimator based on the hydroacoustic counts yielded slightly higher values ( N = 2783, 95% CL 2529–3037), but were still in the same order of magnitude as those from the hydroacoustic and mark–recapture approach. At low run-size, hydroacoustic counting was more time consuming and technically demanding than mark–recapture studies. At the same time, it was non-invasive, provided real-time data on a fine temporal scale, and estimates showed less variability than the Jolly–Seber model. Mark–recapture of fish in spawning streams involved substantial disturbance at a sensitive stage of the life cycle. Hence, hydroacoustics is highly suited for population estimates of threatened potamodromous fishes, where interference needs to be minimized.     

Lantern fish (Benthosema pterotum) resources as a target for commercial exploitation in the Oman Sea

The skinnycheek lantern fish Benthosema pterotum (Myctophidae) represents a virgin resource in the Oman Sea. Because of its potential importance for fishmeal production there has been great interest during the last decade in starting commercial exploitation. Acoustic surveys to estimate stock biomass and trial fishing using pelagic trawls were carried out in Iranian waters of the Oman Sea from 1992 to 1998. Lantern fish schools are found in offshore waters, on the edge of the continental shelf wherever sea depth is more than 100 m. During the day the fish occur in two vertical layers, with the densest schools in the shallower layer (80–130 m), whereas at night B. pterotum occur in a single dispersed layer closer to the surface (mainly at 30–70 m). Biomass estimates range from 1 to 4 million tonnes (t), with an average of 2.3 million t. Densities vary seasonally, with the highest densities recorded in spring (May–June) and lowest in autumn (October–November). The highest densities were seen in the western Oman Sea. Although recommendations can be made with regard to the best fishing seasons, areas and depths, trial catch rates were too low (<30 t daily per boat) to support a commercially viable fishery. Further trials are needed to identify the best gear and vessel size.     

Spatial patterns in the biology of the chokka squid, Loligo reynaudii on the Agulhas Bank, South Africa

Although migration patterns for various life history stages of the chokka squid (Loligo reynaudii) have been previously presented, there has been limited comparison of spatial variation in biological parameters. Based on data from research surveys; size ranges of juveniles, subadults and adults on the Agulhas Bank were estimated and presented spatially. The bulk of the results appear to largely support the current acceptance of the life cycle with an annual pattern of squid hatching in the east, migrating westwards to offshore feeding grounds on the Central and Western Agulhas Bank and the west coast and subsequent return migration to the eastern inshore areas to spawn. The number of adult animals in deeper water, particularly in autumn in the central study area probably represents squid spawning in deeper waters and over a greater area than is currently targeted by the fishery. The distribution of life history stages and different feeding areas does not rule out the possibility that discrete populations of L. reynaudii with different biological characteristics inhabit the western and eastern regions of the Agulhas Bank. In this hypothesis, some mixing of the populations does occur but generally squid from the western Agulhas Bank may occur in smaller numbers, grow more slowly and mature at a larger size. Spawning occurs on the western portion of the Agulhas Bank, and juveniles grow and mature on the west coast and the central Agulhas Bank. Future research requirements include the elucidation of the age structure of chokka squid both spatially and temporally, and a comparison of the statolith chemistry and genetic characterisation between adults from different spawning areas across the Agulhas Bank.     

Spawning aggregations of squid (Sepioteuthis australis) populations: a continuum of ‘microcohorts’

The aim of this study was to determine how size, age, somatic and reproductive condition, abundance and egg production of southern calamary spawning aggregations changed during the spawning season in each of 2 years. During the spawning period in at least one of the years there was a decline as much as 20% in average size, 50% in somatic condition, 28–34% in size-at-age, 26–29% in reproductive status, as well as abundance and reproductive output of the stock declining during the spawning season. However, this change was not a function of the population becoming reproductively exhausted, as the aggregation was composed of different individuals with different biological characteristics. In each month the average age of individuals was ca. 6 mo, indicating that squid that had hatched at different times had entered the spawning aggregations, suggesting that the aggregation was made-up of a succession of microcohorts. Currently, management of many squid populations assumes that there is a single cohort in the aggregation. Therefore, estimating stock biomass at the start of the spawning season cannot be used as the population is constantly changing as micro-cohorts move into the aggregation. An instantaneous estimate of the spawning biomass, independent of fishing activity may be obtained by quantifying the density of deposited eggs. The strategy of individuals with a diversity of life history characteristics coming together in a single spawning aggregation may ensure the phenotypic and genetic diversity required to guarantee successful recruitment of this short-lived species. Therefore, temporally structured protection from harvest throughout the spawning season will ensure maintenance of this population diversity.     

Spatial and temporal variation of abundance,biomass and diversity within marine reserves in the Philippines

Aim The objective of this study was to investigate the influence of protection duration (years of fishing closure) and location (distance from shore) on reef fish diversity. Location Danajon Double Barrier Reef, Bohol, Philippines. Methods Reef fish abundance and size structure, by species, were obtained monthly using replicated underwater visual belt transects (n = 8; 70 × 5‐m belt transects) over 3 years (2002–2005) at eight sites that included six marine reserves and two unprotected reef areas. We analysed species accumulation curves, diversity indices and abundance–biomass comparison (ABC) curves within and across the study sites to assess the influence of protection duration and location. Results Analyses showed that longer protection duration impacted reef fish diversity at both inshore and offshore sites by shifting ABC curves from higher abundance than biomass curves at fished sites to higher biomass than abundance curves at most of the protected sites. Protection duration did not significantly influence either the rate of species accumulation within sites or the 12 diversity indices measured across the study sites. The offshore sites consistently showed higher rates of species accumulation and diversity indices values than inshore sites with similar protection duration. One protected offshore young marine reserve site that has been assessed as the least well‐managed showed patterns more consistent with the fished sites. Main conclusions Analyses showed that protection duration mainly impacted diversity by increasing the dominance of large‐bodied species and enhancing total biomass. Besides protection duration, reserve location influenced species accumulation curves and diversity indices.     

Reproductive characteristics of the ocean leatherjacket, Nelusetta ayraudi

The ocean leatherjacket (Nelusetta ayraudi) is one of the largest members of the family Monacanthidae. Distributed throughout inshore waters around the southern half of Australia, this schooling species supports substantial commercial and recreational fisheries. N. ayraudi do not conform to either of the general reproductive modes reported within the family, but exhibit characteristics of both strong social reproductive behaviour and of being promiscuous and polygynous. Sexual dimorphism, with males and females exhibiting differing colouration and body shapes, and ripe ovaries being an order of magnitude larger than ripe testes, are characteristic of social reproductive behaviour and pair spawning. In contrast, high batch fecundity (mean of 320 oocytes per gram of body weight), similar sizes and ages at sexual maturity (350 mm and 2.5 years respectively) and the formation of large spawning aggregations in offshore waters are characteristic of being promiscuous and polygynous. Similar to many other coastal marine species off the east and west coasts of Australia, N. ayraudi are partial spawners during the austral winter months with spawning restricted to the part of their distribution that is towards the upper area of the prevailing currents. It is hypothesized that N. ayraudi off eastern Australia have evolved a life-history strategy whereby fish move northwards through time, spawning occurs in these more northern waters and the southerly flowing Eastern Australian Current facilitates dispersal of eggs and larvae southwards. The reproductive characteristics described provide various options to fishery managers who wish to enhance the sustainability of the fishery through increased egg production. These include spatial and temporal fishing closures to protect breeding fish during the spawning period, the protection of juveniles through either inshore area closures, improving the selectivity of fishing gears and/or regulated minimum legal lengths.     

Angel sharks (Squatinidae): A review of biological knowledge and exploitation

Angel sharks (Squatina spp.) are distributed in warm temperate to tropical waters around the world. Many species occur in shelf seas and exhibit seasonal inshore–offshore migrations, moving inshore to give birth. Consequently, there can be high spatial overlap of angel shark populations with fisheries and other human activities. Their dorso-ventrally flattened body shape, large size (most species attain >100 cm total length, L_T) and demersal nature means that they may be taken in a variety of demersal fishing gears from birth. Available data indicate that angel sharks typically have a biennial reproductive cycle, with litter sizes generally <20 and the young born at c. 20–30 cm. The biological characteristics of angel sharks render them susceptible to overexploitation, as exemplified by the decline of Squatina squatina from many parts of its former range in the north-east Atlantic and Mediterranean Sea. Currently, half of the 22 recognized extant species of angel shark are classed as Threatened on the International Union for Conservation of Nature (IUCN) Red List (with a further three classified as Data Deficient). Given the biological vulnerability of angel sharks, and that many species are data-limited, the current paper provides a review of available biological information and fisheries data pertaining to this family.     

Population structure of the black seabream Spondyliosoma cantharus along the south-west Portuguese coast inferred from otolith chemistry

The chemistry of black seabream Spondyliosoma cantharus otoliths from three main fishery grounds (Olhão, Sagres and Sesimbra) located along c. 400 km of the Portuguese south and west coasts was examined. Element:Ca ratios were determined in whole otoliths and otolith cores of young adult specimens of 2–3 years of age. Using the data from whole otoliths, it was possible to discriminate among S. cantharus from the three fishing grounds with an average accuracy of 91%. Differences among fishing grounds were significant for all element:Ca ratios, and otoliths from Sagres had significantly higher levels of all ratios compared to the other fishing grounds. In contrast, the chemical composition of the otolith core, representative of the larval stage, showed limited variation among the fishing grounds, with an average discrimination accuracy of only 44%, although the Mg:Ca ratio of the otolith cores was also significantly higher for the Sagres samples. The data suggest that larval stages experienced a homogenous environment consistent with an offshore oceanic spawning. Juveniles appeared to display local residency on the inshore fishing grounds, areas probably characterized by greater environmental heterogeneity. Spondyliosoma cantharus population structure is consistent with distinct local population units that share a spawning ground providing recruits to different coastal fishery areas.     

The life-history strategy of deepwater sculpin, Myoxocephalus thompsoni (Girard), in Lake Michigan: dispersal and settlement patterns during the first year of life

Deepwater sculpin, Myoxocephalus thompsoni (Girard), were sampled from six stations from the 15–100 m depth contours in Lake Michigan between April 1983 and July 1984. In south-eastern Lake Michigan M. thompsoni lay benthic eggs in offshore waters, which hatch between November and August, with peak hatching in March. Abundance of larvae in pelagic samples was higher offshore than inshore, but larval size was greater and development more advanced at inshore stations, indicating an inshore movement after hatching. Larvae reached metamorphosis at 20 mm and settled to the bottom beginning in July. Pelagic larvae 20–40 mm were found in the lower water column at all stations, but newly settled individuals were only captured with bottom trawls at inshore locations (≤60 m depth). Data from ichthyoplankton and bottom trawl samples in 1983 and 1984 indicated that locations for successful settlement of larvae to the bottom extended only as deep as the shallowest fringe of the adult population (> 50 m in 1983). In 1983, maximum density of larvae reached 0.4 individuals m⁻³ by June. Survival from the pelagic larval stage to the demersal young-of-year stage in 1983/1984 was c . 0.1–0.4%. The specific mechanism of mortality at the time of transition to a demersal habit has not been determined.