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Loher T.,International Pacific Halibut Commission | Hobden J.C.,University of Victoria
Fishery Bulletin | Year: 2012

Field experiments were conducted to test the hypotheses that Pacific halibut (Hippoglossus stenolepis) display small-scale spatial structure within longline catches, relative to other species and empty hooks, or within-species based on sex or length. Sequential hook-by-hook inventories, along with length and sex data, were taken at thirty-one survey stations. Two-dimensional spatial statistics were used to test for 1) aggregation, defined as the clustering of individuals within a given demographic of size or sex over small intervals of distance; and 2) segregation, defined as the sequential occurrence of individuals within a given demographic of size or sex, uninterrupted by other observations, irrespective of the distance between individuals. Statistically significant structure was detected within catches that is more commonly associated with fish length than sex. Significant spatial structuring occurred at 60% of all stations tested. Significant aggregation of halibut of legal length for commercial retention (≥82 cm) was detected at 44% of stations and aggregation of sublegal-size halibut was detected at 11%. Male-and female-based aggregations were observed at 22% and 11% of stations, respectively. Significant segregation of females was observed at 20% of stations, male segregation occurred at 8% of stations, and segregation by size at 16% of stations. Understanding small-scale spatial structure within longline catches may help us interpret changes in survey and commercial catch data. If structure is generated by behavior, then observed size-at-age or relative sex-ratios may be biased relative to underlying distributions. Although physical processes such as gape limitation should remain stable over the time, dynamic processes may be spatially and temporally variable.


Thorson J.T.,National Oceanic and Atmospheric Administration | Stewart I.J.,National Oceanic and Atmospheric Administration | Stewart I.J.,International Pacific Halibut Commission | Taylor I.G.,National Oceanic and Atmospheric Administration | Punt A.E.,University of Washington
Marine Ecology Progress Series | Year: 2013

Recruitment is highly variable in marine fishes, and is often estimated using stock-recruit relationships that explain little of the observed variability in recruitment. Researchers have sought for decades to identify environmental indices that are associated with cohort strength, and often use stock assessment estimates of recruitment within secondary regressions to test hypothesized drivers of recruitment variability. This practice is statistically questionable because it fails to acknowledge differences in the precision of recruitment estimates among species and years, as well as covariance between recruitment estimates within a given species. We developed an alternative, statistically rigorous method to estimate an index of cohort strength that is shared among several species while accounting for each single-species stock-recruit relationship. This method simultaneously optimizes multiple stock assessment models with shared cohort strength parameters, while using observation-level fishery data for each species to propagate the precision and covariance of recruitment estimates. The method is demonstrated using data for 8 groundfish species off the US West Coast for which recruitment is relatively well estimated: our model estimated high recruitment during 1990-1991 and 1999-2000, followed by anomalously low recruitment during 2002-2007. The impact of a shared index of cohort strength is demonstrated for 2 additional species with little information about recruitment, yelloweye Sebastes ruberrimus and blackgill Sebastes melanostomus rockfishes, where it decreases the coefficient of variation for recruitment estimates in the most recent modeled year by 40%. The method can be applied to other fishery management regions in the USA and elsewhere, and represents a rigorous method to estimate associations in cohort strength among species within a region. © Inter-Research 2013.


Thorson J.T.,University of Washington | Thorson J.T.,National Oceanic and Atmospheric Administration | Clarke M.E.,University of Washington | Stewart I.J.,National Oceanic and Atmospheric Administration | And 2 more authors.
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2013

Bottom trawl sampling is used to estimate trends in stock abundance for ground fishes worldwide, including Pacific rockfishes (Sebastes spp.). However, trawl sampling efficiency varies spatially and the distribution of ground fish populations may change among easy- and difficult-to-survey areas over time. These concerns have prompted interest in using underwater vehicles (UVs), for which catchability is likely to decrease less in rocky habitats. In this study, we use simulation modeling to evaluate the abundance trends arising from bottom trawl sampling given density-dependent habitat selection and spatially varying catchability. We first demonstrate that relative abundance indices in this case will generally be biased measures of changes in population abundance. We also propose and evaluate a sampling design that combines data from bottom trawl and UV gears. Combined sampling has greater precision than UV sampling, lower bias than bottom trawl sampling, and is robust to moderately violated assumptions regarding sampling strata or spatial catchability. We conclude by recommending future research that could test the assumptions under which combined sampling is a feasible solution to spatially varying catchability.


Seitz A.C.,University of Alaska Fairbanks | Loher T.,International Pacific Halibut Commission | Norcross B.L.,University of Alaska Fairbanks | Nielsen J.L.,U.S. Geological Survey
Aquatic Biology | Year: 2011

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.© Inter-Research 2011.


Galindo H.M.,University of Washington | Loher T.,International Pacific Halibut Commission | Hauser L.,University of Washington
Marine Biotechnology | Year: 2011

The discovery of genetic markers linked to physiological traits in wild populations is increasingly desired for ecological and evolutionary studies, as well as to inform management decisions. However, identifying such markers often requires a large investment of both time and money. Serendipitously, in a recent microsatellite survey, we discovered three out of 16 microsatellite loci that were correlated to the female sex in Pacific halibut (Hippoglossus stenolepis). These three loci were screened in 550 Pacific halibut to determine their accuracy at identifying sex. Genetic assignment successfully identified sex in 92% of individuals from sample collections spanning 3,000 km and 9 years. All but two of 287 females had one copy of a characteristic allele for at least one of the three microsatellite loci, resulting in consistent heterozygote excess in females. This pattern is consistent with the hypothesis that females are the heterogametic sex in Pacific halibut, which thus may have a different sex-determination pattern than the closely related Atlantic halibut (Hippoglossus hippoglossus). A rapid divergence of sex-determining mechanisms could be either a cause or consequence of speciation between Pacific and Atlantic halibut. In either case, the ability to genetically identify sex in individual Pacific halibut provides a new tool for ecological studies, fisheries management, and insight into the evolution of sex determination in flatfish. © 2011 Springer Science+Business Media, LLC.


Stewart I.J.,International Pacific Halibut Commission | Martell S.J.D.,International Pacific Halibut Commission
Fisheries Research | Year: 2014

The Pacific halibut stock assessment has proven to be a particularly challenging application for the estimation of selectivity. Contributing factors include: extremely pronounced temporal changes in length-at-age, a steep vulnerability curve for commonly used hook sizes, a minimum length limit, relatively late (~age 6-10) appearance of fish in survey and fishery data, and geographic heterogeneity in demographic parameters coupled with pronounced spatial trends in population abundance over time and significant ontogenetic migration over the stock range. Historical stock assessments have variously modeled selectivity as a function of length or age, employing nonparametric forms in attempting to account for these various factors. Despite these efforts, a strong retrospective bias in model results occurred during three separate time periods; each of which ultimately required modification of the selectivity parameterization to ameliorate that bias. This paper provides a summary of historical approaches, and the methods employed to address the most recent retrospective pattern. © 2013 Elsevier B.V.


Martell S.,International Pacific Halibut Commission | Stewart I.,International Pacific Halibut Commission
Fisheries Research | Year: 2014

Changes in the observed size- or age-composition of commercial catch can occur for a variety of reasons including: market demand, availability, temporal changes in growth, time-area closures, regulations, or change in fishing practice, to name but a few. Two common approaches for dealing with time-varying selectivity in assessment models are the use of discrete time-blocks associated with an epoch in the history of the fishery, or the use of penalized random walk models for parametric or non-parametric selectivity curves. Time block periods, or penalty weights associated with time-varying selectivity parameters, are subjective and often developed on an ad hoc basis. A factorial simulation-estimation experiment, with discrete or continuous changes in selectivity, is conducted to determine the best practices for modeling time-varying selectivity in fisheries stock assessments. Both the statistical properties of the assessment model and the policy implications of choosing the wrong model are taken into consideration. © 2013 Elsevier B.V.


The fishery for halibut (Hippoglossus stenolepis) in the eastern Pacific is closed during the boreal winter, roughly corresponding to the seasonal spawning of the species. Opening and closing dates for each season are stipulated annually based on economics and biology. Historical surveys and data from electronic tags are analysed to assess the extent to which recent closures have encompassed the annual spawning cycle of the species, as defined by migration to offshore spawning sites, active spawning, and return to feeding areas. These were assessed by calculating mean maximum daily depth profiles for fish exhibiting seasonal migration, calculating the date-specific proportions of the tagged population either migrating to or resident on their feeding or spawning grounds, and examining the temporal distribution of spent and running fish in historical surveys along with evidence of spawning contained in high-resolution tag data. The data indicate that fishery closures over the past 20 years have been consistently too short to protect the entirety of a migration period that begins as early as September and is not substantially completed until May. Additionally, some recent season openings have encroached on the active spawning season. Failure to fully protect spawning migrations may allow seasonal interception fisheries, and the selective removal of early and late spawners could cause changes in stock demographics, restrict effective spawning, and influence long-term stock productivity, especially in the face of environmental variability. © 2011 International Council for the Exploration of the Sea.


Webster R.A.,International Pacific Halibut Commission | Clark W.G.,International Pacific Halibut Commission | Leaman B.M.,International Pacific Halibut Commission | Forsberg J.E.,International Pacific Halibut Commission
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2013

Results of a coastwide tagging study show that ontogenetic migration of Pacific halibut (Hippoglossus stenolepis) continues for larger fish, whereas in recent years the assumption had been that only smaller, younger fish migrated. In 2003-2004, a total of 67 000 Pacific halibut tagged with passive integrated transponder tags were released by the International Pacific Halibut Commission (IPHC) from Oregon to the Bering Sea. Portside scanning recovered over 3000 of these tags. Models were fitted that allowed commercial fishing mortality to be a function of fish length, year, and IPHC regulatory area, while migration probability was a function of area and length. Estimates from the models support the view that exploitation rates were much higher in eastern than western areas prior to the reduction of quotas following new results from a coastwide stock assessment in 2007. We explore possible explanations for differences between tagging and IPHC stock assessment results and note that this research provides confirmation of historical inferences regarding patterns of halibut migration based on conventional tagging.


Loher T.,International Pacific Halibut Commission | Rensmeyer R.,International Pacific Halibut Commission
Reviews in Fish Biology and Fisheries | Year: 2011

Management of Pacific halibut (Hippoglossus stenolepis), a long-lived flatfish, is complicated by possible ontogenic and sex-specific variation in migration. Archival tags promise the ability to help uncover long-term movement patterns at the individual level, if the tags can be retained and recovered from healthy fish. We examined fifteen individuals (69-90 cm fork length) for long-term physiological response to intracoelomic implantation of three types of archival tags: fully internal, internal with right angle protruding light stalk, and internal with straight protruding light stalk. Tags represented 0.05-0.16% of initial fish weights. Fish were reared at 10.8 ± 1.1°C for 59 weeks post-surgery. One fish died after 39 weeks from thermal stress unrelated to the surgical procedure. Temporal variation in behavior of tagged fish was indistinguishable from that of controls (n = 15 tagged, 5 controls). Treatment and control-group fish grew at similar rates. No tag expulsion or physiological response was evident in the individual that died at 39 weeks, but nine of eleven individuals dissected at the end of 59 weeks had developed internal responses. These responses ranged from deposition of fibrous protein and/or calcitic material on tag surfaces to partial or full tag encapsulation in either the visceral peritoneal layer (fully-internal tags) or the intestinal mesenteries (stalk-bearing tags). The responses were within the range reported for other pleuronectids implanted with tags of similar configuration and may have implications for design and interpretation of long-term tagging studies. Encapsulation may reduce the probability of tag recoveries even in the absence of tag expulsion, especially in species eviscerated at sea. © 2010 Springer Science+Business Media B.V.(outside the USA).

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