Raja Rhina Research Articles

Raja rhina

This datasheet on Raja rhina covers Identity.

Rapid age estimation of longnose skate (

There is a paucity of age data for chondrichthyan fishes owing, in large part, to limitations in traditional age estimation methods. Fourier transform near-infrared (FT-NIR) spectroscopy has shown promise as an alternative, more efficient method for acquiring age data from chondrichthyans. However, studies are limited to sharks in the southern hemisphere. We explored FT-NIR spectroscopy to predict age for a batoid species in the northern hemisphere. The longnose skate (Raja rhina) is one of a small number of batoids for which annual band periodicity in vertebral centra has been validated, allowing for traditional age estimation and making it an ideal candidate for this study. We fit a multivariate partial least-square predictive model between FT-NIR spectra collected from vertebral centra and traditional age estimates, and tested model predictive skill by using external validation. Using FT-NIR spectroscopy, we were able to predict age for longnose skates between the ages of 1 and 14 years with precision and bias near equal to those of traditional methods in less than a quarter of the time. These results support potential for FT-NIR spectroscopy to increase the amount of age data available for assessments used to inform the conservation and management of this sensitive group of species.

Bomb radiocarbon analyses validate and inform age determination of longnose skate (Raja rhina) and big skate (Beringraja binoculata) in the north Pacific Ocean

Age and growth estimates based on growth band counts of sectioned vertebrae have been produced for longnose skate (Raja rhina) and big skate (Beringraja binoculata [formerly Raja binoculata]) populations in the Gulf of Alaska, British Columbia and California. Previous growth studies involving estimates from different laboratories in the USA (Alaska Fisheries Science Center, AFSC; Pacific Shark Research Center, PSRC at Moss Landing Marine Laboratories) and Canada (Fisheries and Oceans Canada, DFO) have produced dissimilar results for either species, highlighting the need for development of a consistent age determination protocol and more importantly an age validation study. Archived large specimens of longnose skate and big skate collected in Monterey Bay, CA, in 1980 and 1981 had minimum preliminary age estimates from vertebral growth band counts old enough to suggest that radiocarbon (14C) signals from bomb testing conducted in the late-1960s could be used to establish dates of growth band formation. To this end, we micro-milled skate vertebral thin sections, measured Δ14C using mass spectrometry, and estimated year of growth band formation based on the estimated age from growth band counts using both unstained and stained preparation methods Non-linear random effects modeling, implemented in a Markov Chain Monte Carlo (MCMC) simulation, was used to compare the skate Δ14C data set to a marine teleost otolith reference chronology for the California Current System. Results showed Δ14C measurements for big skate were non-informative as none of the archived samples were old enough for comparison to the reference curve, hence validation of the age estimation approach was not possible. However, for longnose skate, Δ14C data were more informative to fit pulse function models and compare results to the reference chronology. Modeling results indicated longnose skate age estimates based on unstained vertebral thin sections were less biased (overestimated to a smaller degree) than estimates based on stained vertebral thin sections. The degree of bias depended on agency ageing criteria, with the least biased age estimates produced by age readers from the AFSC. The AFSC age estimates had about a 70% probability that age estimates of longnose skate was within+/− 2 years of the expected age based on the radiocarbon assays. We were able to validate the age estimation methodology for longnose skate and establish criteria for growth band counts, which should now be useful to generate region-specific accurate growth and life history parameters required for more reliable stock assessment approaches.

Dietary variability in two common Alaskan skates (Bathyraja interrupta and Raja rhina)

Determining trophic relationships within and among species can provide insight into the structure and function of an ecosystem, and can inform the development of multi-species monitoring and management plans. The goal of this study was to address the need for dietary and trophic information of two common and abundant skates in Prince William Sound (PWS), the Bering skate, Bathyraja interrupta, and longnose skate, Raja rhina. Based on identification and analysis of stomach content data, both species were found to be generalist predators with diets dominated by crustaceans and supplemented with teleosts. The primary source of dietary variability for each species was total length, with spatial variables (i.e., latitude, longitude, and depth) also explaining a large portion of total dietary variability in the Bering skate. However, only a small proportion of the total intraspecific dietary variation was accounted for among the analyzed variables, suggesting substantial individual-based differences in the feeding habits of each species. Trophic level estimates indicated that the Bering skate and longnose skates <100 cm total length (TL) were secondary consumers, whereas longnose skates ≥100 cm TL were more piscivorous, tertiary consumers. Significant interspecific dietary differences were also evident, suggesting trophic separation, likely as a function of skate size. Given their abundance and generalistic feeding behavior, PWS skates can provide a means of monitoring demersal community health; information about their foraging ecology will be valuable in gaining a better understanding of trophodynamics within the PWS food web.

Can observer sampling validate industry catch reports from trawl fisheries?

Stock assessment scientists and fishery managers operate under the necessary assumption that the identities of species and quantities of catch from industry landing reports are known without error. To evaluate this assumption, we compared industry-generated reports of landed catch to independent observer estimates. An observer sampling design for shore-based processing plants was developed and implemented at four plants in Kodiak, Alaska. A total of 13 deliveries from three fisheries were examined. Observers were able to track different portions of the catch and treat them as strata from which to randomly sample or completely enumerate. Differences between observer- and industry-derived species proportions were negligible when measured across the entire study, but differed by fishery. Industry weight in the shallow-water flatfish fishery exhibited a small negative bias not related to processor or species type. Weight differences in complete enumerations for big (Beringraja binoculata) and longnose skates (Raja rhina) were of similar magnitude but in opposite directions, leading to the conclusion that the identification of these species is confused since there is no detection error. Where observers needed to sample, they were able to detect most species in open access fisheries, with a resolution comparable to that of the industry, and were more likely to detect skates than industry. However, in the cooperative rockfish fishery, where tighter controls on the dockside sorting of fish by plant staff are in place, industry reports had enhanced detection of rare species relative to observer sampling. Notwithstanding, differences between data sources remained substantial in strata where the observer sampled even after considering rarity. The results here highlight the utility of using third-party verification to improve data quality of self-reported data, and identified the logistical, database, and analytical challenges to effectively monitor fishery quotas.

Nutritional content, mercury, and trace element analyses of two skate (Rajidae) species in the Gulf of Alaska

Seafood is recognized as an important source of proteins and long chain omega-3 fatty acids. However, one of the primary concerns with seafood consumption is the level of heavy metals, particularly mercury, present in fish tissues, which may influence the demand of certain fishery products. We sampled muscle and liver tissues from 20 big (Beringraja binoculata) and 20 longnose (Raja rhina) skates collected near Kodiak and Cordova, Alaska, and analyzed their nutritional content (protein, moisture and lipid content and fatty acid profiles), heavy metal (mercury, arsenic, cadmium, lead) and trace element (selenium) load. Big and longnose skate muscle was composed of lean protein (14.7%±0.7% SD) with 1.2% (±0.4%) lipids and 83% (±0.8%) moisture. Skate livers were very high in lipids, between 52.5 and 57.5% and had high percentages of omega-3 fatty acids (30.2%). Mercury in these skates had mean levels of 0.21mg/kg, lower than average levels found in Pacific halibut (Hippoglossus stenolepis). Overall, the risk/benefit ratio of consuming skate muscle was slightly positive (3.62%) based on the balance of mercury toxicity and omega-3 fatty acid benefits. Big skates were overall more beneficial to consume, and only longnose skates from Cordova had a negative risk/benefit ratio. These data can be used by the fishing industry to understand current and future market demands for skate products, and to be aware of any health concerns of consuming Gulf of Alaska skates.

Impact of light on catch rate of four demersal fish species during the 2009–2010 U.S. west coast groundfish bottom trawl survey

To determine the influence of light on catch of demersal fish, we examined the relationship between near-bottom light levels, catch rates, and catch probability for four abundant groundfish species well represented in annual bottom trawl surveys on the U.S. west coast: arrowtooth flounder (Atheresthes stomias), greenstriped rockfish (Sebastes elongatus), longnose skate (Raja rhina), and Pacific hake (Merluccius productus). Relative downward irradiance was measured with MK-9 archival tags during annual trawl surveys along the U.S. west coast in 2009 and 2010. Near-bottom light levels were recorded for 818 hauls at depths less than 400m. Significant linear relationships were observed between catch per unit effort (CPUE, kgha−1) and near-bottom light (P<0.05). CPUE of arrowtooth flounder, longnose skate, and Pacific hake was negatively related to near-bottom light. For these species, CPUE decreased 16–22% per unit increase in log10 light (μEm−2s−1). CPUE of greenstriped rockfish increased 39% per unit increase in log10 light. Light, depth, and latitude explained 15–47% of the variance in CPUE for the four species. Catch probability was significantly related to light, depth, latitude, and relative time of day (P<0.05). For all species, catch probability varied inversely with light when depth was less than 200m. At depths of 200–300m, catch probability increased with light for arrowtooth flounder and greenstriped rockfish. Catch probability for Pacific hake decreased slightly at depths greater than 200m while longnose skate was relatively unaffected by light at these depths. We used these relationships to explain the variability in catch rates for individual species within bottom trawl surveys. By influencing the density and distribution of these groundfish species, light can alter catch rates. Furthermore, we found possible herding of greenstriped rockfish, and trawl avoidance by arrowtooth flounder, Pacific hake, and longnose skate.

Spatial segregation in eastern North Pacific skate assemblages.

Skates (Rajiformes: Rajoidei) are common mesopredators in marine benthic communities. The spatial associations of individual species and the structure of assemblages are of considerable importance for effective monitoring and management of exploited skate populations. This study investigated the spatial associations of eastern North Pacific (ENP) skates in continental shelf and upper continental slope waters of two regions: central California and the western Gulf of Alaska. Long-term survey data were analyzed using GIS/spatial analysis techniques and regression models to determine distribution (by depth, temperature, and latitude/longitude) and relative abundance of the dominant species in each region. Submersible video data were incorporated for California to facilitate habitat association analysis. We addressed three main questions: 1) Are there regions of differential importance to skates?, 2) Are ENP skate assemblages spatially segregated?, and 3) When skates co-occur, do they differ in size? Skate populations were highly clustered in both regions, on scales of 10s of kilometers; however, high-density regions (i.e., hot spots) were segregated among species. Skate densities and frequencies of occurrence were substantially lower in Alaska as compared to California. Although skates are generally found on soft sediment habitats, Raja rhina exhibited the strongest association with mixed substrates, and R. stellulata catches were greatest on rocky reefs. Size segregation was evident in regions where species overlapped substantially in geographic and depth distribution (e.g., R. rhina and Bathyraja kincaidii off California; B. aleutica and B. interrupta in the Gulf of Alaska). Spatial niche differentiation in skates appears to be more pronounced than previously reported.

Length–Length and Width–Length Conversion of Longnose Skate and Big Skate Off the Pacific Coast: Implications for the Choice of Alternative Measurement Units in Fisheries Stock Assessment

AbstractMost traditional fisheries models have size or stage–age relationships as their foundation. Two long‐established metrics used are FL and TL. Body length measurements are linked to age, maturity, and fecundity. Obtaining FL or TL measurements are not always convenient, especially for larger fish (&gt;100 cm TL). Two skate species, Longnose Skate Raja rhina and Big Skate R. binoculata, were collected from commercial fishery landings along the coast of Washington State and length–length or width–length conversions were investigated for four metrics of size: TL, tail only length (TOL), interspiracular width (ISW), and interorbital width (IOW). Relationships between TL and alternative measures were examined using model II regression analyses and confirmed a strong linear relationship in all cases (R 2 &gt; 0.94). It is likely that both TL and TOL measures have errors due to worn‐off tail portions; conversely, the ISW is nonlaborious and convenient to obtain. The ISW metric has a smaller coefficient of variation compared with IOW if we assume that both metrics have the same distribution of measurement errors. We recommend ISW as the preferred alternative metric in the assessment of large skate. The ISW will probably yield measurements that are equivalent, or potentially superior, to traditional metrics because the measurement is taken from body parts devoid of worn‐off portions.Received December 30, 2011; accepted June 13, 2013

The complete mitochondrial genome of the Longnose skate: Raja rhina (Rajiformes, Rajidae)

The complete sequence of mitochondrial DNA of a longnose skate, Raja rhina was determined for the first time. It is 16,910 bp in length containing 2 rRNA, 22 tRNA and 13 protein coding genes with the same gene order and structure as those of other Rajidae species. The nucleotide of L-strand is composed of 30.1% A, 27.2% C, 28.5% T and 14.2% G, showing a slight A + T bias. The G is the least used base and markedly lower at the third codon position (5.4%). Twelve of the 13 protein coding genes use ATG as their start codon while the COX1 starts with GTG. As for stop codon, only ND4 shows incomplete stop codon TA. This mitogenome is the first report for a species of the genus Raja, and providing a valuable resource of genetic information for understanding the phylogenetic relationship and the evolution of the genus Raja as well as the family, Rajidae.

Preliminary chemical and nutritional characterization of liver from longnose skates ( Raja rhina)

Skates ( Raja rhina) have recently become a small commercial fishery in Alaska and along the western United States coast. Most of the skate byproduct is discarded or made into meal; therefore, there is opportunity to enhance the utilization for skate byproducts. The objective of this research project was to chemically characterize longnose skate livers. Livers from five long nose skates, caught off the coast of Kodiak, Alaska, were obtained immediately after the fins (called “wings”) had been removed by a commercial processor. Each liver was subjected to the following analysis: proximate composition, minerals, amino acids, α-tocopherol, protein gel electrophoresis, fatty acid profiles, thiobarbituric acid reactive substances (TBARS) and biogenic amines (BA). Livers were composed of lipid (49.6%), moisture (41.0%), protein (10.9%) and ash (0.8%). High levels of α-tocopherol were found in the livers (142 μg/g oil). Fatty acid profile indicated that the lipids extracted from livers contained high concentrations of eicosapentaenoic acid (16.0%) and docosahexaenoic acid (17.7%). Average TBARS values were low at 1.5 μg malondialdehyde/g oil, indicating low levels of lipid oxidation. The lysine content as percent of total amino acids on a weight basis was 6.1% and methionine content was 2.8%. Analysis of BA found putrescine (31 mg/kg liver) and spermine (85 mg/kg liver) present. Preliminary results suggest long nose skate livers as an abundant source of n-3 fatty acids and amino acids.

Using food web model results to inform stock assessment estimates of mortality and production for ecosystem-based fisheries management

Examining food web relationships for commercially important species enhances fisheries management by identifying sources of variability in mortality and production that are not included in standard single-species stock assessments. We use a static mass-balance model to evaluate relationships between species in a large marine ecosystem, the coastal Gulf of Alaska, USA. We focus on food web relationships for four case-study species: Pacific halibut ( Hippoglossus stenolepis ), longnose skate ( Raja rhina ), walleye pollock ( Theragra chalcogramma ), and squids (order Teuthoidea). For each, we present the species’ position within the food web, evaluate fishing mortality relative to predation mortality, and evaluate diet compositions. We find that high trophic level (TL) species, whether commercially valuable (halibut) or incidentally caught (skates), have mortality patterns consistent with single-species assessment assumptions, where fishing mortality dominates natural mortality. However, assessments for commercially valuable (pollock) or incidentally caught (squids) mid-TL species can be enhanced by including food web derived predation information because fishing mortality is small compared with high and variable predation mortality. Finally, we outline food web relationships that suggest how production of species may change with diet composition or prey availability.

Aspects of the Biology and Species Composition of Skates (Rajiformes) from Insular Waters of Southeastern Alaska

Key words: Raja rhina, Bathyraja, reproduc-tive biology, sex ratio, size composition, speciescomposition, Clarence Strait, Dixon Entrance,Gulf of AlaskaDirect and indirect fishing pressure on skates(Order: Rajiformes) has resulted in populationdeclines of exploited stocks and considerableshifts in size structure and species compositionof skate assemblages (Walker and Hislop 1998;Dulvy and others 2000; Rogers and Ellis 2000;Frisk and others 2002). Fishing mortality isespecially deleterious to many skate speciesbecause of their k–selected life history traits,including slow growth, late ages at maturity,and low fecundity (Holden 1974; Cailliet andGoldman 2004). Large skate species are morelikely to exhibit these life history patterns andare therefore typically more susceptible toexploitation than smaller species (Dulvy andReynolds 2002; Ellis and others 2005).Incidental catch of skates is substantial inseveral Gulf of Alaska groundfish fisheries suchas cod longline, halibut longline, and flatfish

First Records of the Whiteblotched Skate (Bathyraja maculata) in the Eastern Gulf of Alaska

The Whiteblotched Skate (Bathyraja maculata) is an elasmobranch species of the family Rajidae that was first described in 1977 from 21 specimens collected throughout the Bering Sea (Ishiyama and Ishihara 1977). Bathyraja maculata is distinctive, with a dorsal surface that varies from gray to dark brown and is punctuated with white to yellow blotches of various sizes. The ventral surface is white to light gray and often blotchy, darkening at the margins of the disc and on the uniformly dark tail, which has broad lateral folds. Nuchal and tail thorns are present and strongly developed, but middorsal, orbital, and scapular thorns are absent. The snout is short and flexible, and the interobital space is flat and wide (Ishiyama and Ishihara 1977; Mecklenburg and others 2002). Female B. maculata reach a maximum total length (TL) of 147 cm (Stevenson and others 2007), with males reported to a maximum size of 132 cm TL (JW Orr, National Marine Fisheries Service (NMFS), Alaska Fisheries Science Center (AFSC), Seattle, WA, pers. comm.). Bathyraja maculata ranges from the northern Sea of Japan and Sea of Okhotsk throughout the Bering Sea and Aleutian Islands (Mecklenburg and others 2002) at depths of 73 m (Fedorov 2000) to 1193 m (Stevenson and others 2007). Its easternmost record in the Aleutian Islands is off Umnak Island (53°24'N, 168041'W; Mecklenburg and others 2002). Bathyraja maculata has been commonly taken in trawl surveys of the western Bering Sea continental slope (Orlov 1998; Dolganov 1999), eastern Bering Sea continental slope (Hoff and Britt 2003, 2005) and Aleutian Islands (Zenger 2004). It also frequently occurs in shallower, continental shelf waters (ca. 100 to 200 m) of the western Bering Sea (Orlov 1998; Fedorov 2000) and Aleutian Islands (Zenger 2004), but appears to be scarce on the broad, eastern Bering Sea continental shelf (Lauth and Acuna 2007a, 2007b). Previous records of B. maculata from the Gulf of Alaska are few and restricted to the Gulf's western margin, where it has been reported at 3 locations during NMFS-AFSC bottom trawl surveys (Stevenson and others 2007, 2008; Fig. 1). The easternmost of these records represents a specimen collected approximately 163 km southwest of Kodiak Island (56°04.8'N, 156034.2'W, 191 m depth) during the summer of 1999 (DE Stevenson, NMFS-AFSC, Seattle, WA, pers. comm.). However, 3 B. maculata were recently collected from the Alexander Archipelago, southeastern Alaska, and are presented here. These records considerably extend the known distribution of B. maculata and suggest that it may be more widespread in the Gulf of Alaska than previously indicated. The Alaska Department of Fish and Game (ADFG) conducts annual longline surveys in Clarence Strait, including US waters of Dixon Entrance and Chatham Strait of the Alexander Archipelago, to assess the condition of Sablefish (Anoplopoma fimbria) stocks in these areas. Gear used during ADFG surveys consists of longlines containing forty-five 13/0 Mustad circle hooks per line, fastened at 2-m intervals and baited with squid. Twenty-five contiguous lines are deployed at established locations of 366 m depth, and gear is fished for 3 to 11 h prior to retrieval. All fishes caught during these surveys, including skates, are enumerated. Typically, only Longnose Skates (Raja rhina) are identified to species, but during the 2006 lower Clarence Strait survey, conducted from 17 to 22 May, skates of all species were distinguished. In addition, photos were taken of WINTER 2008 193

The population dynamics of the longnose skate, Raja rhina, in the northeast Pacific Ocean

The longnose skate, Raja rhina, is the most commonly landed skate species in the northeast Pacific Ocean. It also dominates survey catches in the area. Existing knowledge on skate biology and fisheries suggests that large skate species with late sexual maturation, like the longnose skate, are extremely vulnerable to overfishing, and if overfished they are slow to recover. Historically, in the US Pacific Coast skates have not supported directed fisheries, but have been taken as bycatch in other commercially important fisheries. For the past 13 years, landed catches of skates in waters off Washington, Oregon and California have significantly increased. Using the Stock Synthesis 2 modeling framework, a population model for the longnose skate was developed to assess the current state of the stock and reconstruct its past dynamics. Model results indicate that the longnose skate spawning biomass has slowly declined since 1915; the current stock biomass is estimated as 66% of its unexploited level. Uncertainty in regards to fishery-dependent data and female maturity parameters was explored through sensitivity analyses. The model results indicate that it is important to conduct species-specific identification in the fishery and monitor discard of longnose skate to improve the accuracy of fishery catch data and make assessment conclusions more robust.

Reproductive biology of two commercially exploited skates, Raja binoculata and R. rhina, in the western Gulf of Alaska

This study provides the first detailed information on the reproductive biology of two large and abundant eastern North Pacific Ocean skate species: big skate ( Raja binoculata) and longnose skate ( Raja rhina). Skates were collected in the western Gulf of Alaska during 2005 and 2006 after a targeted fishery for these two species developed around Kodiak Island, Alaska; both species are also taken as by-catch to other fisheries. A total of 307 females (20.1–183.3 cm TL) and 138 males (19.5–143.7 cm TL) R. binoculata, and 354 females (18.1–145.0 cm TL) and 318 males (21.1–135.8 cm TL) R. rhina were collected. Female R. binoculata were found to attain first (e.g., smallest individual) maturity at 125.8 cm TL and 50% at 148.6 cm TL. Males were found to attain first and 50% maturity at 124.0 and 119.2 cm TL, respectively. No gravid female R. binoculata, with egg cases in utero, were encountered during the study. Female R. rhina were found to attain first and 50% maturity at 102.2 and 113.1 cm TL, respectively. Gravid female R. rhina, with egg cases in utero, were recorded throughout the sampling period. Males attained first maturity at 101.0 cm TL and 50% maturity at 102.9 cm TL. Histological analysis did not reveal significant changes in spermatocyst production for either species during the sampling period. Both species appear to be reproductively active year-round as no seasonal cycle was evident. This study provides new and essential information on the reproductive biology of these two commercially targeted skate species.

Field investigation of rare-earth metal as a deterrent to spiny dogfish in the Pacific halibut fishery

Spiny dogfish ( Squalus acanthias) comprise a significant unwanted bycatch on demersal longlines set for halibut, sablefish ( Anoplopoma fimbria), and Pacific cod ( Gadus macrocephalus) in shelf waters of the east and west coasts of North America. Recently, rare-earth magnets and metals have been shown to have deterrent effects on sharks. These effects are likely the result of magnetic or electric fields created by these materials in seawater, which are sensed and avoided by sharks. Our earlier laboratory studies showed that attack rates by spiny dogfish on baits protected with cerium mischmetal (a rare-earth alloy) were reduced and suggested that this metal might reduce unwanted bycatch of spiny dogfish in setline fishing for Pacific halibut ( Hippoglossus stenolepis). We conducted a field study near Homer, Alaska in September 2007 with three hook treatments interspersed on 36 longline sets. These included standard circle hooks used in the halibut fishery, hooks with small pieces of cerium mischmetal attached above the hook, and hooks with a similar (but inert) mild steel piece above the hook. Fewer dogfish were caught on hooks with mischmetal than on either of the two other treatments. Reductions in catch of longnose skate ( Raja rhina) also occurred on hooks protected with mischmetal. However, halibut catch did not increase with protected hooks. Limitations in using mischmetal in commercial operations are expense, hazardous nature, and relatively rapid hydrolysis in seawater.

Age and growth of big skate (Raja binoculata) and longnose skate (R. rhina) in the Gulf of Alaska

In 2003, big skates, Raja binoculata, and longnose skates, Raja rhina, were the target of a commercial fishery around Kodiak Island in the Gulf of Alaska (GOA) for the first time. The sudden development of a fishery for these species prompted the need for improved life history information to better inform fishery managers. Due to the selective nature of the skate fishery, mostly larger individuals were captured. Back-calculation from skate vertebral measurements was used to estimate size-at-age for younger skates. Because back-calculated age-length data within individuals were highly correlated, bootstrap resampling methods were used to test for differences between male and female growth curves. Results from bootstrapping indicated that differences between male and female growth were statistically significant for both species. This investigation indicates that growth of big skates in the GOA (max size 178 cm total length, max age 15 years) is similar to that in California, but different from that in British Columbia. For longnose skates, our GOA results agree with those reported in British Columbia, but were considerably older (max size 130 cm, max age 25 years) than those reported in California, which may not be surprising because longnose skates in the present study were generally larger. This life history information suggests that both big and longnose skates are at risk of unsustainable exploitation by targeted fisheries.

Food habits of the longnose skate, Raja rhina (Jordan and Gilbert, 1880), in central California waters

Feeding studies can provide researchers with important insights towards understanding potential fishery impacts on marine systems. Raja rhina is one of the most common elasmobranch species landed in central and northern California demersal fisheries, yet life history information is extremely limited for this species and aspects of its diet are unknown. Specimens of R. rhina were collected between September, 2002 and August, 2003 from fisheries-independent trawl surveys. Percent Index of Relative Importance values indicated that the five most important prey items in 618 stomachs of R. rhina were unidentified teleosts (31.6% IRI), unidentified shrimps (19.6% IRI), unidentified euphausiids (10.9% IRI), Crangonidae (7.4% IRI), and Neocrangon resima (6.0% IRI). There were significant dietary shifts with increasing skate total length and with increasing depths. Smaller skates ate small crustaceans and larger skates ate larger fishes and cephalopods. With increasing depths, diet included bentho-pelagic teleosts and more cephalopods and euphausiids. The findings of this study are consistent with previous researchers that report similar diet shifts in skate species with size and depth.

Assemblage Structure of Eastern Pacific Groundfishes on the U.S. Continental Slope in Relation to Physical and Environmental Variables

AbstractWe analyzed data from the National Marine Fisheries Service continental slope trawl surveys (1999–2002) to examine patterns in eastern Pacific groundfish assemblage structure in relation to longitude, latitude, depth, temperature, and interannual variation. The slope surveys spanned approximately 32–48°N and 200–1,200‐m depths. We focused on the 26 most abundant species, which made up more than 95% of the catch in terms of biomass. Assemblage structure was strongly correlated with depth and latitude. For the most part, changes in assemblage structure appeared to be fairly continuous, although there were more‐abrupt changes at approximately 500–600 m and in the vicinity of Point Conception and Cape Mendocino, California, and Cape Blanco, Oregon. There was also an interaction between depth and latitude; more northerly sites had deeper scores on the canonical axes than did more southerly sites of comparable depth. Shallower sites also showed more variation in assemblage structure with latitude than did deeper sites. We were able to identify five assemblages on the continental slope. Pacific grenadiers Coryphaenoides acrolepis, giant grenadiers Albatrossia pectoralis, deepsea soles Embassichthys bathybius, longspine thornyheads Sebastolobus altivelis, California slickheads Alepocephalus tenebrosus, and roughtail skate Bathyraja trachura formed a deepwater assemblage with a broad latitudinal range. Southern species that inhabited very shallow waters included splitnose rockfish Sebastes diploproa, spotted ratfish Hydrolagus colliei, English soles Parophrys vetulus, stripetail rockfish Sebastes saxicola, chilipeppers Sebastes goodei, and shortbelly rockfish Sebastes jordani. Shallow‐water, midlatitude species included spiny dogfish Squalus acanthias, longnose skate Raja rhina, Pacific hakes Merluccius productus, rex soles Glyptocephalus zachirus, bigfin eelpout Lycodes cortezianus, and sandpaper skate Bathyraja interrupta. Darkblotched rockfish Sebastes crameri, arrowtooth flounder Atheresthes stomias, and Pacific Ocean perch Sebastes alutus were identified as a shallow, northerly assemblage. The final group—sablefish Anoplopoma fimbria, brown cat sharks Apristurus brunneus, aurora rockfish Sebastes aurora, Dover soles Microstomus pacificus, and shortspine thornyheads Sebastolobus alascanus—occurred at different depths over a range of latitudes and might be best described as a shallow–middepth and midlatitude–southern grouping.