Identification of habitat used for skate egg deposition has been rarely studied or reported worldwide. Four nursery sites for the Alaska skate Bathyraja parmifera, 2 for the Aleutian skate B. aleutica and 2 for the Bering skate B. interrupta were identified along the upper continental slope in the eastern Bering Sea. All sites were located near undersea canyons from 145 to 380 m depth in relatively flat sandy to muddy bottom habitat. Bottom temperatures were relatively constant throughout the year, varying from 3.7 to 4.6°C. Egg case densities varied between nursery sites and were encountered at the Alaska skate nursery in Bering Canyon at densities greater than 800 000 eggs km -2 . Based on egg case composition, sites were predominantly used by a single skate species for egg deposition; however, up to 6 skate species used the habitat commonly. Seasonal sampling indicated that sites were continuously occupied throughout the year, and embryo length composition showed multiple cohorts developing simultaneously. Data from bottom trawl surveys suggest juvenile skates occupy habitats different than nursery sites. The movement of juvenile skates out of nursery habitat after hatching may lessen predation by common predators such as the Pacific cod Gadus macrocephalus and the Pacific halibut Hippoglossus stenolepis.KEY WORDS: Elasmobranch · Skate · Reproduction · Egg case · Undersea canyon · Nursery · Alaska Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 403: [243][244][245][246][247][248][249][250][251][252][253][254] 2010 Lauth & Acuna 2009). Combined these 3 species constitute > 95% of the skate population and biomass in the entire EBS (20 to 1200 m depth) and are the major components of skate bycatch in non-target commercial bottom trawl and longline fisheries (Stevenson 2004). Other important slope-dwelling species in the EBS include the whiteblotched skate B. maculata, the mud skate B. taranetzi, the Commander skate B. lindbergi and the whitebrow skate B. minispinosa (Hoff & Britt 2009).This study focuses on identification of skate nursery habitat for 3 abundant skate species in the eastern Bering Sea. Four nursery sites for the Alaska skate, 2 nursery sites for the Aleutian skate, and 2 nursery sites for the Bering skate ( Fig. 1) are described. Some important biological aspects examined were skate and skate egg case composition, egg case densities and predation on juvenile skates. Nursery habitat parameters include location, temporal use, depth, temperature, bottom topography and associated sessile fauna EasternBering SeaGulf of Alaska P e rv e n e ts C a n y o n P r ib il o f C a n y o n B e r in g C a n y o n MATERIALS AND METHODSNursery habitat identification. Initial locations of nursery sites were identified based on fisheries data, and previous survey trawls in which large numbers of skate egg cases were reported in the catch. Identified sites were subsequently sampled using bottom trawls in an adaptive sampling approach to identify the egg...

The use of more than a single nursery habitat type is examined for oviparous elasmobranchs using data summarized from studies conducted on the Alaska skate Bathyraja parmifera and the Aleutian skate Bathyraja aleutica in the eastern Bering Sea. The eastern Bering Sea skate species use two discrete areas as nurseries, one for egg deposition and a second for newly emergent juveniles. Egg deposition sites were located along the outer shelf and upper slope near canyons in the eastern Bering Sea. Newly emergent juveniles were found along the outer and middle shelf for B. parmifera and deep-slope for B. aleutica, suggesting that habitat used by newly emergent juvenile skates is distinct from habitat used for egg deposition and embryo development. In reviewing many studies on oviparous elasmobranchs, similar patterns emerge of habitat use during their early life history. To distinguish these distinct habitats, appropriate terminology is proposed. Egg case nursery is suggested for areas of egg deposition and juvenile nursery is suggested for areas where juveniles aggregate after emergence. Criteria to describe each habitat type are outlined.

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Embryo development events were correlated with egg-case changes for the Aleutian skate Bathyraja aleutica and the Alaska skate Bathyraja parmifera. Yolk absorption underwent two phases: that of steady absorption during early development and that of rapid yolk absorption during the final development stages. Total length (L(T)) for 50% of the pre-hatching embryos egg-case jelly disappearance was 92.04 mm (range 81-102 mm) and 99.36 mm (range 81-100 mm) for B. aleutica and B. parmifera, respectively, allowing the inner chamber to open to seawater flow. The tail filament underwent three phases of growth: rapid elongation during early development (<100 mm embryo L(T)), stasis of tail filament length during the remainder of embryo development and rapid absorption soon after hatching. Complete tail filament development coincided with the disappearance of egg-case jelly. Clasper buds first developed at embryos >70 mm L(T) for both species and the sex ratio was 1:1 well before hatching. Egg cases that were devoid of an ova or developing embryo were c. 5.0 and 6.5% of the egg cases examined for B. aleutica and B. parmifera, respectively. Measurements showed that egg cases containing only egg jelly were smaller in both width and length than those possessing an ova. Embryo stages were punctuated with distinct events that correlated with egg case changes controlling the internal environment of the developing embryo.

Morphology, time of formation and deposition of the first increment, and rate of increment deposition were determined for sagittae, lapilli, and asterisci from Lost River sucker Deliistes luxatus and shortnose sucker Chasmistes brevirostris. Sagittae and lapilli form on the day of hatch, and asterisci form after 41-52 d in Lost River sucker and after 14-25 d in shortnose sucker. Lapillus increments were validated by using known-age hatchery-raised larvae and wild-caught larvae and juveniles marked by immersion in water with an alizarin complexone concentration of 50 mg/L for 4 h and 18 h, respectively. A transition from relatively wide to relatively narrow increments was sometimes seen during the second week after hatch and appears to be associated with yolk absorption. Because of their readability and conservative growth, lapilli are the preferred otolith structure for age determination of larval and juvenile suckers, whereas sagittae are preferred for nonostariophysan fishes.

Predation on skate eggs by snails was examined for three skate species at seven nursery sites in three regions (north, middle and south) of the eastern Bering Sea. Mean predation levels were 6.46% for the Alaska skate Bathyraja parmifera, 2.65% for the Aleutian skate Bathyraja aleutica and 22.25% for the Bering skate Bathyraja interrupta. Predation levels were significantly higher at the middle and north sites than the south sites for all species combined. Predation levels decreased with increasing egg-case densities at all nursery sites, and the highest predation levels occurred where egg-case densities were very low. Predated egg-case density increased with increasing snail densities across all nursery sites examined. The Oregon triton Fusitriton oregonensis was the most abundant snail species at all nursery sites and displayed ability to drill holes in the egg case of B. parmifera. Holes left by predatory snails in egg cases of B. parmifera were significantly larger, and of different shape at the middle site compared to the south site. Holes in B. parmifera were also significantly larger than those in egg cases of B. interrupta across all sites examined. Egg cases of B. aleutica possess surface spines that cover the egg case and may inhibit predation by snails at a greater rate than that of the B. parmifera and B. interrupta, which have a smoother egg-case surface.

For those marine fish species with specific habitat preferences, a habitat-based assessment may provide an alternative to traditional surveys. We conducted a habitat-based acoustic and stereo image stock assessment survey for rockfishes (Sebastes spp.) on a rocky ridge habitat in the eastern Bering Sea. Video analysis suggested that juvenile and adult rockfishes were more abundant on the seafloor in the rocky ridge area than on the surrounding sandy flats. Over the ridges, the distribution of rockfishes was uniformly low in the water column during nighttime surveys and higher during daytime surveys. The opposite pattern was observed in the video on the seafloor between night (high density) and day (lower density), indicating that fish in the water column during the day moved to the seafloor at night. Mean biomass of adult rockfishes for the rocky ridges was 1.54 Â 10 4 tonnes based on acoustic data. The biomass of juvenile fish was estimated to be 9.2 Â 10 2 tonnes. Utilization of similar survey methodologies on a larger scale might improve assessment of rockfishes not only in Alaska, but also throughout their range where fishery-independent biomass estimates have been difficult to obtain.Résumé : Chez les espèces de poissons marins qui possèdent des préférences spécifiques d'habitat, une évaluation basée sur l'habitat pourrait être une méthode de rechange aux inventaires traditionnels. Nous avons fait un inventaire acoustique et vidéo d'évaluation des stocks des sébastes (Sebastes spp.) basé sur l'habitat dans un milieu de crête rocheuse dans l'est de la mer de Béring. L'analyse vidéo indique que les sébastes jeunes et adultes sont plus abondants sur le fond marin dans la région de la crête rocheuse que sur les plats sablonneux adjacents. Sur les crêtes, la répartition des sébastes est uniformément basse dans la colonne d'eau durant les inventaires de nuit et plus élevée durant les inventaires de jour. Un patron contraire s'observe par vidéo sur le fond marin entre les inventaires de nuit (densité élevée) et de jour (densité basse), ce qui indique que les poissons qui sont dans la colonne d'eau durant le jour se déplacent vers le fond marin la nuit. La biomasse moyenne des sébastes adultes sur les crêtes rocheuses est de 1,54 Â 10 4 tonnes d'après les données acoustiques. La biomasse des jeunes poissons est estimée à 9,2 Â 10 2 tonnes. L'utilisation de méthodes semblables d'inventaire sur une plus grande échelle pourrait améliorer l'évaluation des sébastes, non seulement en Alaska, mais aussi sur toute leur aire de répartition dans laquelle des estimations des biomasses indépendantes de la pêche sont difficiles à obtenir.[Traduit par la Rédaction]