Longline Surveys Research Articles

Using models of social transmission to examine the spread of longline depredation behavior among sperm whales in the Gulf of Alaska.

Fishing, farming and ranching provide opportunities for predators to prey on resources concentrated by humans, a behavior termed depredation. In the Gulf of Alaska, observations of sperm whales depredating on fish caught on demersal longline gear dates back to the 1970s, with reported incidents increasing in the mid-1990s. Sperm whale depredation provides an opportunity to study the spread of a novel foraging behavior within a population. Data were collected during National Marine Fisheries Service longline surveys using demersal longline gear in waters off Alaska from 1998 to 2010. We evaluated whether observations of depredation fit predictions of social transmission by fitting the temporal and spatial spread of new observations of depredation to the Wave of Advance model. We found a significant, positive relationship between time and the distance of new observations from the diffusion center (r2 = 0.55, p-value = 0.003). The data provide circumstantial evidence for social transmission of depredation. We discuss how changes in human activities in the region (fishing methods and regulations) have created a situation in which there is spatial-temporal overlap with foraging sperm whales, likely influencing when and how the behavior spread among the population.

Calibration of pelagic stereo‐BRUVs and scientific longline surveys for sampling sharks

SummaryOur understanding of the ecology of sharks and other highly mobile marine species often relies on fishery‐dependent data or extractive fishery‐independent techniques that can result in catchability and size‐selectivity biases. Pelagic Baited Remote Underwater stereo‐Video Systems (pelagic stereo‐BRUVs) provide a standardized, non‐destructive and fishery‐independent approach to estimate biodiversity measures of fish assemblages in the water column. However, the performance of this novel method has not yet been assessed relative to other standard sampling techniques.We compared the catch composition, relative abundance and length distribution of fish assemblages sampled using pelagic stereo‐BRUVs and conventional scientific longline surveys. In particular, we focused on sharks of the family Carcharhinidae (requiem sharks) to assess the sampling effectiveness of this novel technique along a latitudinal gradient off the coast of Western Australia. We calibrated the sampling effort required for each technique to obtain equivalent samples of the target species and discuss the advantages, limitations and potential use of these methods to study highly mobile species.The proportion of sharks sampled by pelagic stereo‐BRUVs and scientific longline surveys was comparable across the latitudinal gradient.Carcharhinus plumbeuswas the most abundant species sampled by both the techniques. Longline surveys selected larger individuals of the family Carcharhinidae in comparison with the length distribution data obtained from pelagic stereo‐BRUVs. However, the relative abundance estimates (catch per unit of effort) from the pelagic stereo‐BRUVs were comparable to those from 5 to 30 longline hooks.Pelagic stereo‐BRUVs can be calibrated to standard techniques in order to study the species composition, behaviour, relative abundance and size distribution of highly mobile fish assemblages at broad spatial and temporal scales. This technique offers a non‐destructive fishery‐independent approach that can be implemented in areas that may be closed to fishing and is suitable for studies on rare or threatened species.

Prevalence of External Skin Lesions and Polycyclic Aromatic Hydrocarbon Concentrations in Gulf of Mexico Fishes, Post‐Deepwater Horizon

AbstractWe surveyed offshore fish populations in the Gulf of Mexico in 2011 and 2012, following persistent reports of abnormal skin lesions and other pathologies in the aftermath of the Deepwater Horizon oil spill. The incidence of skin lesions in 2011 sampling was most frequent in some bottom‐dwelling species along the continental shelf edge north of the Deepwater Horizon site. Longline surveys revealed that by 2012 the overall frequency of lesions in northern Gulf of Mexico (NGM) fishes in the vicinity of the Deepwater Horizon had declined 53%, with severity also declining. Relatively high concentrations of polycyclic aromatic hydrocarbon (PAH) metabolites (up to 470,000 ng naphthalene equivalents/g bile wet weight), indicative of oil‐related pollution, were found in fish bile in 2011; concentrations of summed PAHs measured in fish liver and muscle were relatively low (<35 ng/g) due to the efficient metabolism of these compounds by teleost fish. Significant declines in bile concentrations of naphthalene and phenanthrene metabolites in Red Snapper Lutjanus campechanus between 2011 and 2012 indicate an episodic exposure to elevated levels of hydrocarbons of petrogenic origin. The composition of PAH parent compounds and alkylated homologs in Red Snapper liver samples was highly correlated with oil collected at the Deepwater Horizon wellhead but was less coherent with other PAH sources in the NGM. The elevated 2011 prevalence of skin lesions in some NGM species was unrelated to surface salinity or temperature anomalies and was not the result of an epizootic observable in our histopathology samples but was positively correlated with PAH concentration. Thus, we fail to reject the null hypothesis that elevated skin lesion frequency is unrelated to PAH exposure from the Deepwater Horizon oil spill.Received August 14, 2013; accepted March 26, 2014

Sexual Segregation of Spiny Dogfish in Fishery‐Dependent Surveys in Cape Cod, Massachusetts: Potential Management Benefits

AbstractCommercial gill‐net and longline surveys were conducted in the Cape Cod, Massachusetts, area to assess whether the proportion of male Spiny Dogfish Squalus acanthias in the catch changes throughout a normal fishing day (an event frequently observed by local commercial longliners) and to identify the associated potential benefits for fishery management in the study area. Sex ratio changes were examined with respect to location (north and south of 42°N), environment (depth, surface and bottom temperatures, and salinity) and fishing (type of gear and time of activity). Spiny Dogfish showed differences in sex ratios (the proportion of males in the catch [PM]) that were related to location and type of fishing gear as well as to depth and time. Higher numbers of fish were caught with longlines, and higher numbers of males were caught in the southern area in inshore shallower waters early in the day. In the northern area, the sex distribution found within sets indicated a higher presence of females and a lower presence of males in shallower inshore waters. Changes in environmental parameters (depth and bottom temperature) were correlated with PM only in the south. A consistent seasonal (summer and early fall) diurnal shift in the sex ratio was found within 10 mi (16.1 km) of the east coast of the Cape Cod peninsula. This shift suggests that the implementation of a seasonal, male‐only directed longline fishery be considered for this location. Such a fishery would enhance the sustainability of the U.S. Atlantic Spiny Dogfish stock by reducing fishing pressure on adult females. The sex ratios observed suggest that sexual segregation in Spiny Dogfish in the Cape Cod area corresponds to female avoidance of males coupled with specific mating and/or feeding behavior by males.Received March 19, 2013; accepted November 20, 2013

Post-release survival and behavior and exposure to fisheries in juvenile tiger sharks, Galeocerdo cuvier, from the South Atlantic

General declines in the abundance of sharks due to intense exploitation rates have been increasing the pressure for mitigating bycatch mortality in longline fisheries. Understanding post-release mortality and behavior of sharks is required to ensure the adequacy of management strategies focusing on the release of live animals. Also, such information is crucial for addressing shark attack mitigation strategies focusing on mechanisms other than shark culling. This study used a combination of conventional tags with acoustic and satellite transmitters to assess both post-release survival and behavior and exposure to fisheries in tiger sharks caught in a longline survey off Recife, Brazil. Although none of the sharks experienced post-release mortality, one episode of presumable natural mortality occurred after 45days at-liberty. The exposure of tiger sharks to fishing pressure in this region seems to be high since at least 23% of the tiger sharks tagged and released off Recife were caught by coastal and oceanic fisheries, most (83%) of which after only 69 (±42)days at-liberty. Upon release, all satellite-tracked sharks moved offshore to deeper waters, usually in the oceanic realm, within an average of 17.72 (±10.99)h, corresponding to a minimum cross-shelf swimming speed of 0.40 (±0.22)m·s−1. The archival data of 3 recovered satellite tags depicted post-release behaviors with exceptional detail, with all such sharks exhibiting a yo-yo movement pattern with increasing depths until the shelf break was reached. Some sharks consistently spent a period of 12 (±2)days in deep-diving before returning to shallower waters, presumably from the continental shelf, which could correspond to a species-specific behavioral response to hooking stress. Tagged sharks did not return to the area where they were caught and it would be unlikely that they would return before several months because most of them traveled through great distances to northward latitudes. Altogether, tiger sharks caught alive in longline gear seem to experience negligible post-release mortality if they are adequately released, thus an eventual mandatory release of live animals could be effective to improve the conservation of the species. Also, recently-released tiger sharks should not be expected to further interact with the same fishing gear that caught them for some time, or to further pose any threat to beach users in Recife, where an abnormally high shark attack rate is verified since 1992.

Killer Whale Depredation and Associated Costs to Alaskan Sablefish, Pacific Halibut and Greenland Turbot Longliners

Killer whale (Orcinus orca) depredation (whales stealing or damaging fish caught on fishing gear) adversely impacts demersal longline fisheries for sablefish (Anoplopoma fimbria), Pacific halibut (Hippoglossus stenolepis) and Greenland turbot (Reinhardtius hippoglossoides) in the Bering Sea, Aleutian Islands and Western Gulf of Alaska. These interactions increase direct costs and opportunity costs associated with catching fish and reduce the profitability of longline fishing in western Alaska. This study synthesizes National Marine Fisheries Service observer data, National Marine Fisheries Service sablefish longline survey and fishermen-collected depredation data to: 1) estimate the frequency of killer whale depredation on longline fisheries in Alaska; 2) estimate depredation-related catch per unit effort reductions; and 3) assess direct costs and opportunity costs incurred by longliners in western Alaska as a result of killer whale interactions. The percentage of commercial fishery sets affected by killer whales was highest in the Bering Sea fisheries for: sablefish (21.4%), Greenland turbot (9.9%), and Pacific halibut (6.9%). Average catch per unit effort reductions on depredated sets ranged from 35.1–69.3% for the observed longline fleet in all three management areas from 1998–2012 (p<0.001). To compensate for depredation, fishermen set additional gear to catch the same amount of fish, and this increased fuel costs by an additional 82% per depredated set (average $433 additional fuel per depredated set). In a separate analysis with six longline vessels in 2011and 2012, killer whale depredation avoidance measures resulted in an average additional cost of $494 per depredated vessel-day for fuel and crew food. Opportunity costs of time lost by fishermen averaged $522 per additional vessel-day on the grounds. This assessment of killer whale depredation costs represents the most extensive economic evaluation of this issue in Alaska to date and will help longline fishermen and managers consider the costs and benefits of depredation avoidance and alternative policy solutions.

Reproduction of the sandpaper skate, Bathyraja kincaidii (Garman 1908) in the eastern North Pacific

The sandpaper skate, Bathyraja kincaidii, is commonly taken as bycatch in trawl fisheries off central California. The impact of fisheries exploitation on this population is unknown, however little is known about its biology, including its reproductive life history. To determine reproductive aspects of B. kincaidii, 506 individuals were collected from commercial trawls, museum collections, and monthly trawl and longline surveys conducted by the National Marine Fisheries Service. Males ranged from 112 to 635 mm TL, whereas females ranged from 113 to 610 mm TL. Males reach first maturity at 440 mm TL and at ~3 years, whereas females reach first maturity at 450 mm TL, approximately 1 year later. Size and estimated age at 50 % maturity were similar between sexes: 492 mm TL and 7.5 years for males and 467 mm TL and 7.1 years for females. Gonadosomatic index (GSI) and hepatosomatic index (HSI) values indicate a continuous reproductive cycle, with a seasonal peak in the summer and fall, and a resting period following egg case deposition.

Biological Aspects of Swordfish, Xiphias gladius Linnaeus, 1758, Caught During Tuna Longline Survey in the Indian Seas

Distribution, abundance and biology of swordfish, Xiphias gladius, of the Indian seas were investigated by analyzing the data gathered during tuna longline surveys conducted by the Fishery Survey of India (FSI). Study undertaken during the period from 2004 to 2010 revealed swordfish Catch Per Unit Effort of 0.02 numbers in 100 hooks from the Arabian Sea; 0.01 from Bay of Bengal and 0.02 form the Andaman and Nicobar waters. About 16% of the specimens caught were juveniles and the sex ratio of smaller fishes was in favour of males, whereas, larger specimens were mostly females. Length-weight relationship established indicated slightly positive allometric growth of the species and length based models revealed difference in growth between sexes. The growth parameters estimated for females were: asymptotic length (L∞) = 311.11 cm, growth coefficient (K) = 0.17/yr and age at zero length (t0) = –0.53 yr, whereas, the growth parameters estimated for males were: L∞ = 243.79 cm, K = 0.22/yr and t0 = –0.37 yr. Diet was dominated by finfishes and cephalopods, while crustaceans were recorded rarely. Sthenoteuthis oualaniensis was the dominant prey species, followed by Paralepis sp. Spawning area was identified in the Lakshadweep waters from where mature females with hydrated oocytes were caught during December to April. Size at 50% maturity for females was estimated at 164.03 cm, which is reached at about four years of age. Mean batch fecundity was 4.5 million, while the relative fecundity was 37.5 hydrated oocytes per gram of body weight and the diameters of mature oocytes were in the range of 0.9-1.6 mm. Our results provide preliminary information on the abundance, growth and biology of this species in the Indian seas which should be useful to fishery managers.

Killer whale (Orcinus orca) depredation effects on catch rates of six groundfish species: implications for commercial longline fisheries in Alaska

AbstractPeterson, M. J., Mueter, F., Hanselman, D., Lunsford, C., Matkin, C., and Fearnbach, H. 2013. Killer whale (Orcinus orca) depredation effects on catch rates of six groundfish species: implications for commercial longline fisheries in Alaska. – ICES Journal of Marine Science, 70: 1220–1232. Killer whale (Orcinus orca) depredation occurs when whales damage or remove fish caught on longline gear. This study uses National Marine Fisheries Service longline survey data from 1998–2011 to explore spatial and temporal trends in killer whale depredation and to quantify the effect of killer whale depredation on catches of six groundfish species within three management areas in Alaska: the Bering Sea, Aleutian Islands and Western Gulf of Alaska. When killer whales were present during survey gear retrieval, whales removed an estimated 54–72% of sablefish (Anoplopoma fimbria), 41–84% of arrowtooth flounder (Atheresthes stomias) and 73% (Bering Sea only) of Greenland turbot (Reinhardtius hippoglossoides). Effects on Pacific halibut (Hippoglossus stenolepis) and Pacific cod (Gadus macrocephalus) were significant in the Western Gulf only with 51% and 46% reductions, respectively. Overall catches (depredated and non-depredated sets) for all groundfish species significantly impacted by killer whale depredation were lower by 9–28% (p &lt; 0.05). Effects on shortspine thornyhead (Sebastolobus alascanus) catches were not significant in any management area (p &gt; 0.05). These results provide insight into the potential impacts of killer whale depredation on fish stock abundance indices and commercially important fisheries in Alaska and will inform future research on apex predator–fisheries interactions.

Seasonal abundance, philopatry and demographic structure of Caribbean reef shark (Carcharhinus perezi) assemblages in the north-east Exuma Sound, The Bahamas

The Caribbean reef shark (Carcharhinus perezi), an abundant coral-reef-associated apex predator, is one of the most economically and ecologically important, yet least studied species of large shark in the greater Caribbean region. The relative abundance and population structure of C. perezi off Cape Eleuthera, The Bahamas, was surveyed by standardised longline surveys from May 2008 to October 2011, which resulted in the capture of 331 sharks. Abundance peaked in the summer and was lowest during the winter. Females were 1.6 times more abundant than males and the assemblage was dominated by immature female sharks (45.5 %). The abundance of mature male and female sharks peaked a month apart in June and August, respectively. All 331 sharks were tagged and released with 15.4 % being recaptured after periods at liberty between 5 and 1,159 days (Mean = 333.4 ± 42.7 SE). The mean distance between tagging and recapture was 1.77 km for recaptures in excess of 6 months, indicating seasonally stratified philopatry in this species. C. perezi inhabiting Bahamian waters have developed complex habitat use patterns that are both spatiotemporally and demographically segregated, most probably in response to the large and diverse habitat mosaic available on the Bahamas Banks compared to contemporary study sites. This study represents an important step in understanding the spatiotemporal population structure of C. perezi and illustrates the potential for studies examining behavioural plasticity in response to environmental variation and anthropogenic disturbance.

Plastic debris ingested by deep-water fish of the Ionian Sea (Eastern Mediterranean)

Debris has been recognized as a global environmental problem including within deep habitats. From 26 fish species (1504 specimens) caught in the Eastern Ionian Sea during deep-water long-line surveys, plastic debris was found in 24 individuals of Galeus melastomus (3.2%) and single individuals of Pteroplatytrygon violacea, Squalus blainville, Etmopterus spinax, and Pagellus bogaraveo. The occurrence of debris among their food was infrequent. Ingested debris included primarily plastics (86.5%) and to a lesser extent pieces of metal and wood. Among ingested plastics, fragments of hard plastic material constituted the highest proportion (56.0%), followed by plastic bag fragments (22.0%), fragments of fishing gears (19.0%) and textile fibers (3.0%). Among the species with ingested debris, G. melastomus swallowed all debris categories; P. violacea and S. blainville ingested plastic bag fragments, whereas pieces of hard plastics were found in E. spinax and P. bogaraveo.

Abundance and Distribution of Sharks in Northeast Florida Waters and Identification of Potential Nursery Habitat

AbstractSharks are considered top predators in many marine ecosystems and can play an important role in structuring community ecology. As a result, it is necessary to understand the factors that influence their abundance and distribution. This is particularly important as fishery managers develop management plans for sharks that identify areas that serve as essential fish habitat, especially nursery habitat. However, our understanding of shark habitat use in northeast Florida waters is limited. The goal of this study was to characterize the abundance and distribution of sharks in northeast Florida estuaries and to examine the effect of abiotic factors on shark habitat use. A bottom longline survey conducted from 2009 to 2011 indicated that 11 shark species use the estuarine waters of northeast Florida during the summer months. Atlantic Sharpnose Sharks Rhizoprionodon terraenovae, Blacktip Sharks Carcharhinus limbatus, and Bonnetheads Sphyrna tiburo were the most abundant species and made up 81.4% of the total catch. Site, month, and bottom water temperature were the most important factors determining the presence and abundance of sharks and suggest both regional and seasonal variations in the use of northeast Florida waters. Depth, salinity, and dissolved oxygen were also important factors. Our data show that these waters serve as a nursery for Atlantic Sharpnose and Blacktip Sharks, with young‐of‐the‐year and juveniles being present in the summer months. Limited tag–return data reveal that juvenile sharks remain in these waters throughout the summer and that some return in subsequent summers. This is the first study to characterize the abundance and distribution of sharks and identify potential nursery areas in northeast Florida estuaries.Received October 10, 2012; accepted March 7, 2013

Natural or Artificial? Habitat-Use by the Bull Shark, Carcharhinus leucas

BackgroundDespite accelerated global population declines due to targeted and illegal fishing pressure for many top-level shark species, the impacts of coastal habitat modification have been largely overlooked. We present the first direct comparison of the use of natural versus artificial habitats for the bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species - one of the few truly euryhaline sharks that utilises natural rivers and estuaries as nursery grounds before migrating offshore as adults. Understanding the value of alternate artificial coastal habitats to the lifecycle of the bull shark is crucial for determining the impact of coastal development on this threatened but potentially dangerous species.Methodology/FindingsWe used longline surveys and long-term passive acoustic tracking of neonate and juvenile bull sharks to determine the ontogenetic value of natural and artificial habitats to bull sharks associated with the Nerang River and adjoining canals on the Gold Coast, Australia. Long-term movements of tagged sharks suggested a preference for the natural river over artificial habitat (canals). Neonates and juveniles spent the majority of their time in the upper tidal reaches of the Nerang River and undertook excursions into adjoining canals. Larger bull sharks ranged further and frequented the canals closer to the river mouth.Conclusions/SignificanceOur work suggests with increased destruction of natural habitats, artificial coastal habitat may become increasingly important to large juvenile bull sharks with associated risk of attack on humans. In this system, neonate and juvenile bull sharks utilised the natural and artificial habitats, but the latter was not the preferred habitat of neonates. The upper reaches of tidal rivers, often under significant modification pressure, serve as nursery sites for neonates. Analogous studies are needed in similar systems elsewhere to assess the spatial and temporal generality of this research.

Evaluating the Performance of Vertical Longlines to Survey Reef Fish Populations in the Northern Gulf of Mexico

AbstractA common critique of many stock assessments is the lack of fishery‐independent abundance indices and age composition data. Such data streams are essential in evaluating population trajectories that are derived largely from harvest and age composition of landings. For example, high scientific uncertainty in the most recent stock assessment of Gulf of Mexico red snapper Lutjanus campechanus resulted from a conflict between trends in fishery‐dependent and fishery‐independent data. Because sample sizes for the latter data were an order of magnitude lower, resolution of the conflicting trends was even more problematic. Recognizing the need for cost‐effective expansion of fishery‐independent data in the region, we evaluated the performance of vertical longline surveys for sampling reef fish within a large artificial reef zone in the northern Gulf of Mexico. Specifically, we (1) determined species composition and the length frequency of red snapper (the dominant species captured) as a function of hook size and bait type within our survey area during 2010; (2) evaluated the effect of different soak times on catch for various hook types (a combination of hook size and bait type); and (3) utilized our results to test the effect of artificial reef type on red snapper CPUE and mean size. During March–November 2010, we conducted 532 vertical longline sets, capturing 1,217 red snapper that ranged from 184 to 827 mm FL. Mean FL of red snapper differed among hook sizes, with 3/0 and 8/0 hooks sampling smaller fish than 11/0 hooks. Soak time trials revealed a significant effect of soak time on CPUE, with peak catch rates observed at 5 min. As habitat area increased, the mean size and CPUE of red snapper increased. We conclude that our vertical longline is an effective gear for sampling red snapper, and we recommend protocols to maximize its utility and standardize its use.

Comparing Deep-Sea Fish Fauna between Coral and Non-Coral “Megahabitats” in the Santa Maria di Leuca Cold-Water Coral Province (Mediterranean Sea)

Two experimental longline surveys were carried out in the Santa Maria di Leuca (SML) cold-water coral province (Mediterranean Sea) during May–June and September–October 2010 to investigate the effect of corals on fish assemblages. Two types of “megahabitat” characterized by the virtual absence of fishing were explored. One was characterized by complex topography including mesohabitats with carbonate mounds and corals. The other type of megahabitat, although characterized by complex topographic features, lacks carbonate mounds and corals. The fishing vessel was equipped with a 3,000 m monofilament longline with 500 hooks and snoods of 2.5 m in length. A total of 9 hauls, using about 4,500 hooks, were carried out both in the coral megahabitat and in the non-coral megahabitat during each survey. The fish Leucoraja fullonica and Pteroplatytrygon violacea represent new records for the SML coral province. The coral by-catch was only obtained in the coral megahabitat in about 55% of the stations investigated in both surveys. The total catches and the abundance indices of several species were comparable between the two habitat typologies. The species contributing most to the dissimilarity between the two megahabitat fish assemblages were Pagellus bogaraveo, Galeus melastomus, Etmopterus spinax and Helicolenus dactylopterus for density and P. bogaraveo, Conger conger, Polyprion americanus and G. melastomus for biomass. P. bogaraveo was exclusively collected in the coral megahabitat, whereas C. conger, H. dactylopterus and P. americanus were found with greater abundance in the coral than in the non-coral megahabitat. Differences in the sizes between the two megahabitats were detected in E. spinax, G. melastomus, C. conger and H. dactylopterus. Although these differences most probably related to the presence-absence of corals, both megahabitats investigated play the role of attraction-refuge for deep-sea fish fauna, confirming the important role of the whole SML coral province as a refuge area from fishing.

Assessing Adequate Sampling Levels with Time-Series Resampling of Fishery-Independent Bottom Longline Surveys of the U.S. Gulf of Mexico

Assessing Adequate Sampling Levels with Time-Series Resampling of Fishery-Independent Bottom Longline Surveys of the U.S. Gulf of Mexico

Reproductive biology of black hakes (Merluccius polli and M. senegalensis) off Mauritania

Black hake, Merluccius polli and M. senegalensis, are demersal species that inhabit the continental shelf and upper slope off Morocco, Mauritania, and Senegal, and are targeted by Spanish trawlers and bottom longliners. Merluccius senegalensis generally occurs at shallower depths than M. polli, although both species share a wide bathymetric range. Due to their external resemblance and overlapping occurrence at certain depths, both species are mixed in catches and are commonly marketed as Merluccius spp. From 2003 to 2009 biological samplings were performed in Mauritanian waters for numerous specimens of M. polli and M. senegalensis during 15 commercial fishing trips, seven experimental longline surveys, and three trawl surveys carried out within the black hake depth range and mainly during their reproductive periods. Data sets for 1334 fishing stations were thus gathered. The georeferenced macroscopic analysis of the female-assigned maturity stages was combined with the monthly evolution of the gonadosomatic index and with the time series of roe landings from 1984 to 2006 to determine the potential spawning areas, depths, and seasons for both species, showing an intense spawning period from November to February. Size at first maturity was estimated for both species in separate spawning periods and was higher for M. polli females (44 cm) than for M. senegalensis females (39 cm). Sex ratios were also determined by length classes (1 cm) for the experimental survey data and were analyzed according to fishing gear and latitude. The results showed a general predominance of females throughout the depths explored, both for M. polli (1:1.7) and M. senegalensis (1:1.5). Histological samples of ovaries and spawning progression data were jointly analyzed and indicated an apparent spawning delay for the deeper hake, M. polli (October), in comparison with the shallower hake, M. senegalensis (September).

Validating the use of baited remote underwater video surveys for assessing the diversity, distribution and abundance of sharks in the Bahamas

Baited remote underwater video surveys (BRUVS) are a novel, non-invasive method of generating relative abundance indices for a number of marine species, including sharks. This tech- nique has become increasingly prevalent in shark ecology literature, but has yet to be formally vali- dated against more traditional shark survey methods. BRUVS and longline surveys were conducted over 4 seasons from summer 2008 to spring 2009, in 3 habitat zones in the waters off Cape Eleuthera, The Bahamas. By the end of the project, both techniques generated similar values of species richness; however, longline surveys reached these values faster and with fewer replicates than BRUVS. Over- all there was a significant positive correlation of relative abundance between the 2 survey tech- niques, but when analysed on a species-by-species basis, correlations were not statistically signifi- cant for the less abundant species. This suggests a threshold level of abundance, below which relative abundance estimates generated by the different survey techniques do not agree. In addition, this study identified shortcomings in the ability of BRUVS to accurately identify the species, size and sex of individuals captured on video due to the variable behaviour of the sharks in front of the cam- era; however, technological improvements incorporated into contemporary studies are identified which might improve data quality. BRUVS offer a non-invasive alternative to longline surveys for monitoring broad trends in the relative abundance of sharks, although in the format tested in this study they may lack the resolution required to approach finer scale, intra-species studies.

The Shark Assemblage at French Frigate Shoals Atoll, Hawai‘i: Species Composition, Abundance and Habitat Use

Empirical data on the abundance and habitat preferences of coral reef top predators are needed to evaluate their ecological impacts and guide management decisions. We used longline surveys to quantify the shark assemblage at French Frigate Shoals (FFS) atoll from May to August 2009. Fishing effort consisted of 189 longline sets totaling 6,862 hook hours of soak time. A total of 221 sharks from 7 species were captured, among which Galapagos (Carcharhinus galapagensis, 36.2%), gray reef (Carcharhinus amblyrhynchos, 25.8%) and tiger (Galeocerdo cuvier, 20.4%) sharks were numerically dominant. A lack of blacktip reef sharks (Carcharhinus melanopterus) distinguished the FFS shark assemblage from those at many other atolls in the Indo-Pacific. Compared to prior underwater visual survey estimates, longline methods more accurately represented species abundance and composition for the majority of shark species. Sharks were significantly less abundant in the shallow lagoon than adjacent habitats. Recaptures of Galapagos sharks provided the first empirical estimate of population size for any Galapagos shark population. The overall recapture rate was 5.4%. Multiple closed population models were evaluated, with Chao Mh ranking best in model performance and yielding a population estimate of 668 sharks with 95% confidence intervals ranging from 289–1720. Low shark abundance in the shallow lagoon habitats suggests removal of a small number of sharks from the immediate vicinity of lagoonal islets may reduce short-term predation on endangered monk seal (Monachus schauinslandi) pups, but considerable fishing effort would be required to catch even a small number of sharks. Additional data on long-term movements and habitat use of sharks at FFS are required to better assess the likely ecological impacts of shark culling.

Seasonal abundance and distribution of pelagic and demersal fishes in southeastern Alaska

We quantified seasonal and interannual variability of pelagic and demersal fishes available to marine predators in southeastern Alaska focusing on prey of the Steller sea lion ( Eumetopias jubatus). Estimates of abundance were determined from echo integration mid-water trawl and demersal longline surveys. The dominant species were walleye pollock ( Theragra chalcogramma) (average biomass 9057 t), Pacific hake ( Merluccius productus) (1715 t), Pacific herring ( Clupea pallasii) (1176 t), Pacific halibut ( Hippoglossus stenolepis) (average catch rate 524 kg per 1000 hooks), Pacific cod ( Gadus macrocephalus) (177 kg per 1000 hooks), sablefish ( Anoplopoma fimbria) (120 kg per 1000 hooks), and sandpaper skate ( Bathyraja interrupta) (26 kg per 1000 hooks). Of these species, seasonal differences in species abundance were detected for walleye pollock ( p = 0.03), Pacific cod ( p = 0.001) and sablefish ( p < 0.001) with walleye pollock the most abundant and widespread species year-round. Herring, hake, and juvenile (120–350 mm) and adult (>350 mm) pollock are pelagic species. Adult and juvenile pollock and hake were found in open water, while herring, young-of-the-year (<120 mm) and the smallest juvenile pollock were found in bays. Herring of all ages concentrate in dense schools. Pollock and hake form scattered layers throughout open water with juvenile pollock shallower than adult pollock and adult pollock shallower than hake. Halibut, sablefish, skates, Pacific cod and arrowtooth flounder are demersal species; sablefish were deeper than the other demersal species. These seasonal and annual changes in prey availability affected prey selection of sea lions which shifted their diet in response.