A combination of larval behavior and physical factors influence the spatial patterns of settlement of marine organisms. Of particular importance to the settlement process is the blend of passive transport and active responses to water flow near the settlement habitat. Field experiments with the naked goby Gobiosoma bosc, a benthic oyster reef fish, indicated that larvae aggregate in low-flow areas on the downcurrent sides of rocks, and shift position with changing flow directions. Larger aggregations of larvae were found in downcurrent positions at rocks that created larger low-flow zones, and during parts of the tidal cycle with higher ambient flow velocities. Settlement occurred in a highly aggregated pattern that reflected larval distributions. Most settlement measured in a field experiment was adjacent to downcurrent sides of rocks rather than in other positions near rocks, or away from structures that would decrease downcurrent flow velocities. These results suggest that the active response of fish larvae to either direct or indirect effects of flow on reefs may be important to fine-scale spatial patterns of settlement. Because zooplankton densities downcurrent of rocks were sinular to, or lower than, densities upcurrent and lateral to rocks, spatial distnbutions of prey are unlikely to explain larval distributions. Instead, active preference for low-flow areas may enable fish larvae to maintain their position on oyster reefs, the preferred settlement habitat.

Ecological studies, including those focusing on coastal eutrophication, vary in the emphasis they place on species‐level vs. ecosystem‐level processes. The degree of variation among interacting species in their response to perturbations to the physical environment is likely to be important in determining when species‐ or population‐level processes will strongly affect attributes measured at higher levels of ecological organization. We conducted mesocosm and small‐scale laboratory experiments to determine how low oxygen affects predation rates in a zooplankton–fish larvae–larval predator food web typical of mesohaline areas in the Chesapeake Bay. Dissolved oxygen concentrations in bottom waters of the Chesapeake Bay decline during summer to levels that can be physiologically stressful or lethal to animals dependent on aerobic respiration. Our results indicate that the effects of low oxygen on trophic interactions vary among interacting pairs of species in the food web studied. Low but nonlethal dissolved oxygen concentrations greatly increased predation on fish larvae (mostly naked goby Gobiosoma bosc) by sea nettles (the scyphomedusan jellyfish Chrysaora quinquecirrha) but decreased predation by juvenile striped bass (Morone saxatilis). Predation by a single predator, the sea nettle, increased for fish larvae, decreased for fish eggs (Anchoa mitchilli), and was significantly but not strongly affected for copepods (mostly Acartia tonsa) at low dissolved oxygen concentrations. Changes in predator–prey interactions reflected variation among species in their physiological tolerance to low oxygen and the effects of low oxygen on the escape behavior of prey, as well as on swimming and feeding behaviors of predators. Because of the variation in effects on trophic interactions, low dissolved oxygen has the potential to cause major alterations in the relative importance of different pathways of energy flow in the Chesapeake Bay and in other estuarine systems.

Loher, T., and A. Seitz. 2008. Characterization of active spawning season and depth for eastern Pa-cific halibut (Hippoglossus stenolepis), and evidence of probable skipped spawning. J. Northw. Atl. Fish. Sci., 41: 23–36. doi: 10.2960/J.v41.m617 The eastern Pacific halibut (Hippoglossus stenolepis) fishery is prosecuted over a nine-month season with a provision to cease harvests if stock declines to historically-observed minimum spawn-ing biomass. The industry has requested to extend fishing into winter, but little information exists regarding potential impacts on spawning aggregations or effective spawning biomass. A strictly an-nual spawning cycle is presumed, but some adults fail to undertake the offshore migration associated with continental slope spawning. We examined depth records of halibut tagged with Pop-up Archival Transmitting (PAT) tags for evidence of offshore seasonal migration (n = 72). For tags that were physically recovered (n = 16) we identified the occurrence of abrupt (~100 m) mid-winter ascents, believed to be egg release. The active spawning season, defined by occurrence of these rises, lasted from 27 December–8 March, at bottom depths of 278–594 m. Eighteen percent of tagged halibut remained onshore. Thirty-one percent of fish with detailed archival records did not exhibit spawnin

Characterizing migratory behaviours contributes to the sustainable management of marine fishes by resolving stock structure and identifying the timing and locations of events within fish life cycles. The migratory behaviour of Atlantic halibut (Hippoglossus hippoglossus) in the Gulf of St. Lawrence (GSL), Canada was characterized over an annual cycle using pop-up satellite archival tags (n = 15). Daily probability density functions of individual halibut positions were estimated using a geolocation model specifically developed to track demersal fish species in the GSL. Reconstructed migration routes (n = 8) revealed that Atlantic halibut displayed seasonal migrations, moving from deeper offshore waters in the winter to shallower nearshore waters in the summer. Variability in migratory behaviours was observed among individuals tagged at the same location and time. One individual resided year round in the vicinity of the tagging site, three individuals displayed homing behaviour, and four individuals did not return to the tagging site. The identification of presumed spawning rises for two individuals suggested that spawning of Atlantic halibut occurred in the GSL. Although based on a limited number of individuals, these results suggest that Atlantic halibut in the GSL forms a philopatric population, supporting the current separate management of this stock from the adjacent Scotian Shelf and southern Grand Banks stock.

No abstract

Understanding and identifying the genetic mechanisms responsible for sex-determination are important for species management, particularly in exploited fishes where sex biased harvest could have implications on population dynamics and long-term persistence. The Pacific halibut (Hippoglossus stenolepis) supports important fisheries in the North Pacific Ocean. The proportion of each sex in the annual harvest is currently estimated using growth curves, but genetic techniques may provide a more accurate method. We used restriction-site associated DNA (RAD) sequencing to identify RAD-tags that were linked to genetic sex, based on differentiation (FST) between the sexes. Identified RAD-tags were aligned to the Atlantic halibut (Hippoglossus hippoglossus) linkage map, the turbot (Scophthalmus maximus) genome, and the half-smooth tongue sole (Cynoglossus semilaevis) genome to identify genomic regions that may be involved in sex determination. In total, 56 RAD-tags (70 single nucleotide polymorphisms) were linked to sex, and 3 RAD-tags were identified in only females. Sex-linked loci aligned to 3 linkage groups in the Atlantic halibut (LG07: 7 loci, LG15: 1 locus, and LG24: 1 locus), 3 chromosomes in the turbot (LG12: 13 loci, LG01: 1 locus, and LG05: 1 locus), and 1 chromosome in the half-smooth tongue sole (ChrZ: 9 loci). Results add support to the hypothesis that Pacific halibut genetic sex is determined in a ZW system. Two sex-linked loci were further developed into sex identification assays, and their efficacy was tested on individuals that had been morphologically sexed. The accuracy of each assay on its own was 97.5% compared to morphological sex.

Pop-up archival transmitting (PAT) tags were used to study the fall migration of halibut in the Gulf of Alaska (GOA). We tagged 6 Pacific halibut Hippoglossus stenolepis on summer feeding grounds in the eastern GOA and another 6 in the western GOA from June 13 to August 6, 2002. The tags were programed to be released from the fish on January 15, 2003, at the height of the winter spawning season: 10 tags successfully detached, transmitted archived environmental data (depth and temperature), and generated accurate latitude-longitude coordinates shortly after pop-up; 2 tags deployed off SE Alaska were lost. The tags revealed that 6 fish had moved a considerable distance (> 200 km) between tagging and pop-up, and all of these had moved northward to some extent. The longest of the observed migrations was from the southern Alaska Peninsula to Yakutat Bay, a linear displacement of 1153 km; 4 fish showed little evidence of geographic displacement, exhibiting migrations that ranged only from 30 to 69 km. Although 2 fish had moved inshore by the end of the tagging period, all other fish had moved offshore regardless of their overall migration distance. The precise timing of offshore movements varied, beginning as early as August and as late as January. These observations generally corroborate conventional tagging, indicating migration of halibut toward winter spawning grounds in the northern GOA, and movement of fish to deep water in fall. However, no single stereotypic migration behavior was apparent, and a variety of vertical movement patterns and temperature profiles were observed. Halibut spent most time in waters of 5 to 7°C, but experienced temperatures ranging from 2.6 to 11.6°C. Depth observations ranged from 0 to 736 m, with summertime activity concentrated in depths from 0 to 400 m, and halibut that exhibited offshore movement were typically observed at 300 to 700 m by mid-winter. Vertical movement (short-period changes in depth) varied among fish and over time, with some fish displaying little vertical activity, others displaying short periods of activity, and still others displaying considerable activity throughout their time at liberty.

Currently, it is assumed that eastern Pacific halibut Hippoglossus stenolepis belong to a single, fully mixed population extending from California through the Bering Sea, in which adult halibut disperse randomly throughout their range during their lifetime. However, we hypothesize that hali but dispersal is more complex than currently assumed and is not spatially random. To test this hypo thesis, we studied the seasonal dispersal and behavior of Pacific halibut in the Bering Sea and Aleutian Islands (BSAI). Pop-up Archival Transmitting tags attached to halibut (82 to 154 cm fork length) during the summer provided no evidence that individuals moved out of the Bering Sea and Aleutian Islands region into the Gulf of Alaska during the mid-winter spawning season, supporting the concept that this region contains a separate spawning group of adult halibut. There was evidence for geographically localized groups of halibut along the Aleutian Island chain, as all of the individuals tagged there displayed residency, with their movements possibly impeded by tidal currents in the passes between islands. Mid-winter aggregation areas of halibut are assumed to be spawning grounds, of which 2 were previously unidentified and extend the species' presumed spawning range ∼1000 km west and ∼600 km north of the nearest documented spawning area. If there are indeed independent spawning groups of Pacific halibut in the BSAI, their dynamics may vary sufficiently from those of the Gulf of Alaska, so that specifically accounting for their relative segregation and unique dynamics within the larger population model will be necessary for correctly predicting how these components may respond to fishing pressure and changing environmental conditions. Aquat Biol 12: [225][226][227][228][229][230][231][232][233][234][235][236][237][238][239] 2011 Aleutian Islands. It is assumed that adult halibut feed in shallow, nearshore areas during the summer, undertake a spawning migration to deeper water during winter, and return to their summer grounds during spring (Dunlop et al. 1964, Best 1981. In the Southeast Bering Sea (SEBS), spawning appears to be concentrated in relatively discrete winter spawning grounds near the edge of the continental shelf in the Bering Canyon and the Pribilof Canyon ( Fig. 1) (St-Pierre 1984). After spawning, egg and larval stages drift pelagically in the Bering Sea gyre for approximately 6 mo (Skud 1977, St. Pierre 1989 and then settle in nearshore areas (Thompson & Van Cleve 1936). After settling, it is thought that juvenile halibut conduct contranatant migrations to the area in which they were spawned to maintain population stationarity (Skud 1977, Hilborn et al. 1995.From the 1930s through the 1950s, at least 3 stocks of halibut were recognized, 1 in the Bering Sea and 2 in the Gulf of Alaska (GOA) (Fukuda 1962, Skud 1977. After this period, research indicated extensive intermingling of halibut among areas, and it was assumed that there was only a single stock of halibut. This research was based, in part, on...