The study of "spatial population structure" is an important branch of fisheries ecology that seeks to understand the role of early life-history, habitat, behavior, and environmental forcing on the distribution and recruitment patterns of marine organisms; in this case, halibut in the northeast Pacific and southeast Bering Sea.  That's quite a mouthful, but more generally stated, the research centers on how life history traits affect distribution and abundance in both space and time.  That is, at any given point in time, a species is very rarely (if ever) found in equal abundance across its geographic range.  Rather, animals are typically found in patches of high population density surrounded by regions where relatively few individuals occur.  Similarly, population abundance does not remain constant over time, but often cycles between “good” years and “bad” years.  These are fairly simple observations: fishermen have know for thousands of years that fish and shellfish are found on separate grounds scattered along the coast, and that fishing is leaner in some years than in others.  The number of grounds, where they are found, and the manner in which they are positioned relative to one another  (i.e., the population's “spatial structure”) will be different depending on what species you're dealing with.  Spatial population structure is shaped by a species’ habitat requirements over its lifetime, the nature of the landscape that it inhabits, and interactions with members of its own species and with other species in the ecosystem.  For instance, animals may be found in a particular location while they are small, young, and vulnerable to predators, then move out onto important feeding grounds as they become older, larger, and more voracious.  As adults they may move off these feeding grounds on a seasonal basis to congregate on different grounds for mating and spawning.  At each stage in their life-history, habitat requirements may be very different: they may need habitat that provides shelter while young, habitat that provides adequate food supplies and favorable growth conditions when older, and that facilitates congregation and egg-hatching during the spawning season.  For any particular species, the patchwork pattern that the population assumes in time and space will reflect these life-history requirements, their relative importance, and where within the landscape each requirement can be met.

     In order for a species to persist, each age-class must be able to locate appropriate habitat and other members of its species at the appropriate time.  As humans who desire to harvest those animals, it is important that we understand the species' needs and movement patterns, so that we can avoid activities that might compromise spawning potential, disrupt movement between habitat patches, or isolate local groups from the larger population in ways that make them vulnerable to local depletion or collapse.  In order to fully appreciate changes in halibut abundance over time, we need a better understanding of the factors that generate strong year-classes, how the population utilizes habitat at different ages, how animals move between areas on a seasonal and interannual basis, and how the population responds to fishing pressure and environmental changes.  Furthermore, because the environment is an ever-changing beast, we need to realize that the manner in which a stock is structured today may not be the way in which it was structured in the past, nor the way in which it will be structured in the future.  We need to keep an eye on changes in the environment and habitat, so that we can understand and predict and how these physical changes translate into changes in abundance and distribution of the stocks that we harvest.

     The projects abstracted below have been designed to comprise an integrated program to study  migration, site-fidelity and spatial recruitment patterns of halibut in the eastern Pacific.  The individual projects are constructed to represent discrete efforts with explicit objectives.  But these projects are also designed with links to one another, and to existing IPHC research projects, in order to create broader understanding than isolated individual projects would be able to provide.  In addition, the program can be expanded in future years to more completely address the importance of behavior, habitat requirements, ecological interactions, and environmental effects on the Pacific halibut population and the fishery it supports.
 
 

Analysis of spatial recruitment dynamics in Pacific halibut using otolith elemental fingerprints: Phase I

Previous IPHC research suggests that important settlement and nursery grounds for the eastern Pacific halibut population are located primarily in shallow coastal waters, extending from Dixon Entrance in British Columbia to the Bristol Bay region of the southeastern Bering Sea.  Following ~2 years of residence on the nursery grounds, juveniles are believed to migrate in a southeasterly direction, arriving on the fishing grounds as 4-5 year-olds.  While this model of movement is supported by IPHC tagging studies, little information exists regarding the average distance that juveniles migrate or the extent to which various fishing grounds are supplied by specific nursery areas or are populated by a mixture of individuals reared throughout their geographic range.  The answers to these questions may have important ramifications on future management of the directed commercial and sport fisheries, as well as management of halibut bycatch in other fisheries.  In particular, the industry and the Commission remain concerned that juvenile bycatch mortality incurred in the Bering Sea may adversely impact Area 2A and 2B fisheries.  One specific goal of the proposed research is to determine the origin of legal-sized Area 2A-B halibut.  In addition, knowing the relative importance of each nursery will enhance our ability to predict the impact of coastal development and resource extraction on regional recruitment patterns.  For example, if a number of individual fishing grounds are primarily dependent upon a single nursery to generate future recruitment, then the impact of activities that alter habitat quality or survival of juvenile halibut in the source nursery will be felt disproportionately by fishers on the affected ground(s).  Likewise, localized bycatch has the potential to adversely affect different areas of the fishery to different degrees, depending on juvenile movement patterns.  During the first phase of this project, otoliths will be collected from juvenile halibut at at a number of nurseries in the Gulf of Alaska, and will be chemically analyzed to determine if their microelemental composition (their "Otolith Elemental Fingerprint", or OEF) varies with location, thereby generating a "natural tag" that is unique to each nursery.  If unique OEFs are apparent, we will conduct more extensive sampling in nurseries from Dixon Entrance through the southeast Bering Sea in order to develop a regional nursery OEF profile database for the population. These profiles will be used in future years so that otoliths extracted from adult fish can be analyzed to reveal each fish’s nursery origin, allowing us to determine which areas(s) of coastline serve as nursery grounds for various regions of the fishery.

Project schedule:  Field work for Phase I began in the summer of 2002 and was expanded in 2003 to focus on southeast Alaska and questions of temporal stability in OEFs within sites.  We will continued collecting samples opportunistically in 2004 and re-visit our temporal-stability site in 2004.  Other field work has been postponed due to funding issues.  Chemical analysis of the samples began in December of 2003, and a second round is scheduled for June of 2004; we expect that a third series of chemical analyses will take place in fall of 2004.

Analysis of seasonal onshore-offshore movement patterns of Pacific halibut using Satellite Tags

Present day halibut fisheries harvest fish throughout the continental shelf of Canada and the US on the species’ summer feeding grounds.  During the winter months, most of these fish depart the relatively shallow waters of the shelf to aggregate and spawn in deeper waters along the shelf-edge, at spawning grounds that stretch from at least the Queen Charlotte Islands through the Bering Sea and westward.  This general pattern of seasonal migration has been recognized for decades, if not centuries, but a number of questions regarding seasonal behavior remain unanswered.  At exactly what time of the year do halibut begin their migrations, how far do they travel between feeding and spawning grounds, and how long does the journey take?  Do fish remain within their local coastal region to spawn, simply moving offshore of their feeding grounds, or do they travel substantial distances along the coast to reach their final destination?  Once fish reach the spawning grounds, do they remain near the bottom, or move up into the water column to feed?  Until quite recently, these were questions that were exceedingly difficult to answer, but recent advances in tag technology are providing a tool that can be used to uncover some of these mysteries.  Electronic Pop-up Satellite-transmitting Archival Tags (PSATS*) can record temperature, depth, and a number of other water-column parameters while attached to fish.  The tags are programmed to release from the fish on a pre-determined date, float to the surface, and emit a satellite signal that indicates the tag's location, and downloads all of the temperature and depth data to the satellite.  The result is a record of the fish’s spawning location, along with important environmental and behavioral data throughout the fish’s time at liberty.  The IPHC tagged ~12 adult halibut during the 2002 summer setline survey in the Gulf of Alaska.  In addition, a collaborator associated with USGS and the University of Alaska, Fairbanks (Mr. Andy Seitz), tagged 12 additional halibut in the southeastern Bering Sea during during an IPHC-sponsored charter.  All tags deployed in 2002 were programmed to release during the winter of 2003, and information collected from the tags are enhancing our knowledge of seasonal movements patterns and behavior.  An IPHC internal report is in preparation that will fully detail the Gulf releases, and Andy is preparing a similar document detailing the results of the St. Paul releases.  Both reports will be linked to this page when available, but in the mean time you can use the link below to access a brief project update.  Additionally, Andy and his colleagues have published the results of preliminary work that they conducted during 2001, and that document can be accessed with the second link.

The project is ongoing, and during the 2004 survey we will be deploying an additional 24 tags along the Aleutian Chain: 12 will be deployed near Attu Island, and the other 12 just north of Atka and Amlia Islands.  These tags will be programmed to pop up during February of 2005, further enhancing our understanding of halibut ecology in the Bering Sea.

*To learn more about PSATs, click here.  NOTE: this link does not constitute an endorsement of Wildlife Computers Inc., or any of their products.

Genetic population structure of Pacific halibut assessed via mitochondrial DNA and nuclear microsatellite diversity

The eastern north Pacific halibut resource is presently managed under the assumption that a single panmictic population (i.e., a fully mixed population in which members from all geographic regions regularly interbreed) exists from California through the eastern Bering Sea.  This assumption rests largely upon studies that indicate northwesterly larval drift throughout the Gulf of Alaska and into the Bering Sea, balanced by southeasterly migration of juveniles and adults over broad geographic expanses.  In addition, limited genetic studies have failed to demonstrate significant difference between northern and southern stock components.  Thus, Pacific halibut in the eastern Pacific Ocean are treated as a single unit stock with regard to reproduction and recruitment, and managed as a series of regulatory areas with respect to harvest guidelines.  However, there is reason to hypothesize that population structure is more complex, and that the Bering Sea may represent a functional stock component that could be managed independently.  Oceanographic data suggest that larvae spawned in the southeast Bering Sea should remain in the Bering Sea gyre, and the juveniles that migrate from Bristol Bay into the Gulf might simply represent fish that were spawned south of the Alaska Peninsula and were transported into the Bering Sea as larvae.  Previous genetic studies have failed to sample the Bering Sea appropriately and have used genetic markers that lacked the resolution required to identify stock units. No previous research has analyzed fish captured on the spawning grounds, the point in the annual movement cycle that actually defines the genetic stock unit.  During this project, tissue samples will be collected from halibut on the spawning grounds at 3 locations across the range: the Queen Charlotte Islands in British Columbia,  the Portlock Bank of the north-central Gulf of Alaska, and the Misty Moon Ground (Pribilof Canyon) in southeast Bering Sea.  Genetic differentiation will be assessed using mitochondrial DNA and nuclear microsatellites, and results from the two markers compared.  Samples from the Aleatian Islands and western Bering Sea will be included in the analyses if they can be obtained.

Project schedule:  We began isolating microsatellites and establishing lab protocols during the summer of 2002, in collaboration with Dr. Lorenz Hauser at the University of Washington's Marine Molecular Biotechnology Laboratory (MMBL).  This lab work utilized tissue samples collected from commercial landings in the Bering Sea (Adak and St. Paul Islands), Alaska (Petersburg), and Oregon (Newport).   We intended  to collect tissue samples from the spawning grounds during the winter of 2003 but were unable to secure vessels for that work.  Instead, a broad suite of genetic samples from summer feeding grounds were collected through our setline survey and port sampling programs during the summer of 2003, and the winter sampling was delayed until January and February of 2004.  Those winter charters were successful and we now have a considerable archive of adult tissue samples.  Laboratory analysis at UW-MMBL is expected to begin in late summer of 2004.

Group-level breeding site fidelity of Pacific halibut assessed using otolith elemental fingerprints

The establishment of regulatory and statistical areas in fishery management relies heavily upon the concept of the "unit stock"; i.e., that the fish found in each regulatory area represent a functionally discrete breeding population whose dynamics are not substantially influenced by events that occur in neighboring regions.  Defining stock components is sometimes a difficult task, often approached through genetic analyses.  However, while it is clear that groups of fish that are genetically different from one another represent separate stocks, the converse is not necessarily true.  Spatially-structured populations often contain regional units whose population dynamics operate in relative isolation, and which might lend themselves to regional management strategies, but which may demonstrate little genetic divergence from neighboring regions.  In particular, a very small amount of genetic exchange may be sufficient to maintain homogeneity between two regions (e.g., a few individuals every hundred years), especially at loci that display low mutation rates.  Genetic population structure is typically established by isolation mechanisms that are measured in the thousands to tens of thousands of years, whereas the population dynamics most important to fisheries are observed across year-classes, and are described on interannual and decadal time-scales.  Species that home to discrete breeding grounds throughout their lifetime may be vulnerable to local depletion and warrant a management strategy that recognizes changes in stock structure within each breeding area.  During this project we will collect otoliths from fish captured on 3 different breeding grounds and analyze their chemical composition to determine if otoliths collected on different spawning grounds posses unique, identifiable signatures that suggest long-term breeding site fidelity.  Analyses will be conducted separately on males and females of similar age to determine whether they differ, suggesting sex-specific movement patterns over their lifetimes, or posses similar elemental fingerprints that suggest complementary life histories.

Project schedule:  Elementally "clean" otoliths were collected from the spawning grounds aboard the same charters conducted for the project outlined above ("Genetic population structure of Pacific halibut assessed via mitochondrial DNA and nuclear microsatellite diversity").  In addition, we have begun steps to begin a program of regular clean otolith collection during our summer setline surveys.  However, we do not know when we will be able to begin the chemical analysis of the otoliths, given present work loads and funding constraints.  Since the chemical signatures within the otoliths are stable, the timing of the analyses is non-critical.
 

Thermal habitat preferences of Pacific halibut and the potential influence of hydrographic variability on a local coastal fishery

In this project we seek to determine whether average sea bottom temperature (SBT) and its variability over time correlates with catch rates in the Pacific halibut fishery of St. Paul Island in the southeast Bering Sea.  This fishery experienced a marked reduction in harvest between the 2001 and 2002 fishing seasons, prompting concern that resource abundance in the area may be in decline relative to the larger Bering Sea stock.  While local depletion may be a factor in reduced harvest rates, the cause of such depletion remains unknown.  One possible explanation for reduced local abundance relative to the surrounding areas is that changes in ocean conditions have made the shallow water areas near the island less favorable for adult halibut feeding and growth.  In particular, we hypothesize that adult halibut inhabit water masses within a preferred range of mean temperatures, and that temporal variability in water temperature effect halibut distribution and local catch rates.  We will examine relationships between long-term catch per unit effort (CPUE) and decadal-scale changes in ocean conditions using fishery logbook data from the International Pacific Halibut Commission (IPHC) regional temperature records from the US National Oceanic and Atmospheric Administration.  Shorter-term (inter- and intraannual) relationships between CPUE and SBT are being examined by deploying temperature recording devices on commercial longline gear during the course of the 2003 and 2004 fisheries.  In addition, we will use behavioral information retrieved from our ongoing archival tagging studies to compare fishes’ thermal preferences to the temperatures observed by the fishery.  The information gained by this study will aid us in better understanding movement and distribution patterns of halibut in the shallow coastal waters of the Pribilof Islands, and potentially serve as a model for understanding local population dynamics throughout western Alaska.

Project schedule:  A pilot study was conducted during 2002 with the cooperation of the Central Bering Sea Fishermen's Association (CBSFA) to determine the feasibility of deploying temperature loggers on commercial setline gear and to collect baseline data.  Given the success of this pilot study, a proposal was submitted to the North Pacific Research Board (NPRB) to expand the scope and duration of the study, and NPRB funds were awarded to extend the project through 2004.  Loggers were deployed by the fleet in 2003, and will be deployed in 2004 as well.  We hope to continue with long-term monitoring after the NPRB grant period has ended.

A project website has been established for the dissemination of data and reports related to this research.  It is relatively new and we haven't posted a lot on it yet, but the original project proposal can be found there and more will be made available as the work progresses.  That website address is http://www.pribilofresearch.org/, and in the mean time you can view some other documents with the links below.