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New Sablefish Research at the Auke Bay Laboratory

(Quarterly Report for Jul-Aug-Sep 1999)

Michael Sigler, Dean Courtney, Jonathan Heifetz,
Chris Lunsford, Nancy Maloney, and Thomas Rutecki

Sablefish (Anoplopoma fimbria) is the highest valued finfish per pound in Alaska commercial fisheries.  Due to its rich oil content, it is exceptionally flavorful and an excellent fish for smoking. Because sablefish resembles cod, it commonly is called blackcod, though the species does not belong to the codfish family but instead is a member of the Anoplopomatidae family, a group of fishes confined to the North Pacific Ocean. Sablefish are long-lived, and individuals have been estimated to live longer than 90 years.

In Alaska offshore waters, sablefish is managed by the National Marine Fisheries Service (NMFS) under North Pacific Fishery Management Council (NPFMC) Fishery Management Plans. Recent harvests have been about 15,000 to 20,000 metric tons per year.  The Auke Bay Laboratory (ABL) of the Alaska Fisheries Science Center (AFSC) is the primary entity responsible for sablefish research and assessment, with partnerships with the AFSC’s Resource Assessment and Conservation Engineering and Resource Ecology and Fishery Management Divisions.  General areas of sablefish research have been longline surveys, population modeling, stock assessment, and tagging of age-1+ and adult sablefish.  This article examines new areas of sablefish research conducted at the ABL that include incorporating fishery catch rates into annual stock assessments, investigating the life history of young-of-the-year sablefish, electronic tagging of adult sablefish, and estimating tag reporting rates.

Fishery catch rate analysis

Sablefish populations in the Gulf of Alaska have been monitored annually through longline surveys since 1979. Survey catch rates indicate that sablefish abundance has decreased in recent years, but members of the fishing industry have expressed concern because their catch rates have remained strong in some areas and contradict the observed decline in survey catch rates.  The observed decline in survey catch rates has led to decreased fishery quotas, thus reducing fishermen’s individual fishing quotas (IFQs) and the amount of sablefish they can harvest.  Recent work at the ABL has focused on an analysis of commercial fishing data to determine if the fishery and survey show similar trends in sablefish catch rates.

Fishery data are collected by members of the North Pacific Groundfish Observer Program and from commercial fishing vessel logbooks.  The observer program has collected data since 1990. The sablefish logbook program supplies data on catch and fishing location to supplement the observer data.   The sablefish logbook program initially was a voluntary effort established by fishing industry representatives and the ABL in 1997.  Besides the voluntary sablefish logbooks, a groundfish logbook has long been required for all vessels over 60 feet in length, but the information in this logbook was not usable to document catch rates because of lack of detail.  In 1999, the logbook required for vessels over 60 feet in length was improved to better document longline fishery catch rates. The observer program plus the expanded logbook program now provide more data to better estimate trends in fishery catch rates.  Establishment of the sablefish logbook program shows both a commitment from the industry to collect fishery information and a commitment by NMFS to include this information in the stock assessment.

To estimate these trends, we computed a standardized fishery catch rate for each NPFMC management region and year.  Catch rates were standardized by longline set to account for differences in hook spacing, which can vary greatly among vessels.  Standardizing the data lets us compare the fishery and survey data from different regions and years.  Catch rates are expressed as the number of pounds caught per hook.

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Figure 1.   Average CPUE (pounds/hook) by region for sablefish longline survey and fishery, 1990-98.

The highest catch rates for both fishery and survey longlines have been in the West Yakutat and East Yakuta/Southeast areas (Figure 1 above).  Fishery catch rates increased dramatically from 1994 to 1995 in all Gulf of Alaska regions.  The increase is  probably due to the change in 1995 from the short open-access fishery, often called the “derby” fishery, to the 8-month  season for the IFQ fishery.  The fishery and survey catch rate trends from 1995 to 1998 are similar in all Gulf of Alaska areas except West Yakutat.  In West Yakutat, the survey catch rate has declined steadily since 1996; the fishery catch rate, however, increased from 1996 to 1997, followed by a decrease in 1998.

The analysis of catch rate trends is an important step in incorporating both survey and fishery data into the management process.  The fishery catch rate data from this analysis have been added to the sablefish assessment model.  Analysis of the fishery catch rate data also provides a broader view of catch rates through the season and will assist in better understanding the status of the sablefish population in Alaska.

Young-of-the-year abundance

The sablefish fishery is dependent on the recruitment of young fish into the fishable population.  Successful recruitment appears to be most influenced by ocean conditions during early life.  However, little is known about the early life history, growth, and diet of young sablefish in the North Pacific Ocean. The peak spawning period for sablefish in the eastern Gulf of Alaska is thought to occur from late February through March along the continental slope in waters deeper than 300 m.  The eggs develop in deep water for about 2 weeks until they hatch.  The larvae actively swim to the surface after they begin feeding, and in Southeast Alaska and British Columbia, the juveniles appear in nearshore waters by fall.  During the period from first-feeding to nearshore residency, young sablefish are thought to be particularly vulnerable to changes in ocean conditions such as currents and temperature which may affect growth and survival.

In 1995 ABL scientists began a feasibility study of sampling juvenile sablefish, especially young-of-the-year (fish less than 1 year old).  Study objectives were to collect abundance, distribution, age, growth, and diet data on young-of-the-year sablefish and eventually predict future year-class strength.  After the 1995 feasibility study, we decided that a juvenile sablefish survey was feasible. The juvenile sablefish survey has been conducted annually since 1995 during summer from a platform of opportunity: the chartered fishing vessel used for the sablefish longline survey.  The juvenile sablefish survey is conducted by fishing a small-mesh floating gillnet at night, when the longline vessel is not occupied with normal fishing operations.  Catch and length of all captured fish species are measured at sea.  A portion of the juvenile sablefish and juvenile salmon catches are brought back to the laboratory.  Age, growth, and diet are determined, and the salmon are also checked for tags or marks identifying their origin.

Five years of data have been collected from the juvenile sablefish survey through 1999.  Seven gillnet sets were completed during the feasibility study of 1995, and about 35 sets were completed each year during the 1996-99 surveys. The study results showed that fish grew from about 10 cm fork length in early July to about 18 cm fork length by late August.  Few juvenile sablefish have been caught before mid-July, but they are commonly caught during the remaining survey period through late August (Figure 2 below). Juvenile sablefish catches were variable each year, and only 2 years, 1995 and 1999, stand out.  Juvenile sablefish were caught in all gillnet sets only in 1995, compared to about 40% of the sets in other years.  Catch rates were the lowest of all years in 1999.

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Figure 2.  Log-transformed sablefish catch (number +1) per net
by day of year from the surface gillnet survey of the Gulf of Alaska
and Aleutian Islands reigon, 1995-98.

Catching sablefish in all gillnet sets in 1995 suggests that the 1995 year class is above average strength.  Catch rates for this year class also were above average in the 1997 and 1998 longline surveys.  Although the 1998 gillnet catches do not stand out, other information implies that the 1998 year class is above average. Large numbers of age 1+ sablefish have been found throughout the waters off Southeast Alaska and near Cordova in Prince William Sound during  summer 1999.  Previous studies conducted by the ABL have shown that the presence of large numbers of juvenile sablefish in Southeast Alaska waters in a given year often indicate a strong year class.

We also conducted a study to determine which gear type would be better suited for use  if a vessel dedicated to only sampling juvenile sablefish became available. During 1998, we compared surface rope trawl and floating gillnet sampling methods for juvenile sablefish.  Unlike the young-of-the-year sablefish survey where the gillnet is fished during darkness, this study fished the gillnet for young-of-the-year sablefish during daylight. We determined that the surface rope trawl towed from a large trawler would be the gear of choice to conduct a large-scale, young-of-the-year sablefish survey of the Gulf of Alaska because more stations could be sampled per day.  One possible approach is to sample 100 nautical mile-long transects lying perpendicular to shore at selected sites in the Gulf of Alaska.

Young-of-the-year growth

Young-of-the-year sablefish collected during the gillnet surveys also were retained for growth studies. Otoliths, bony structures in the inner ears of a fish, often contain a record of daily growth of a juvenile fish, much like yearly growth rings visible in trees or in otoliths of adult fish.  The goal of our young-of-the-year growth research was to determine if otoliths of young Alaska sablefish contain a daily record of growth, and if so, to reconstruct the early growth history of sablefish from their otoliths.

Of the three types of otoliths in fish, we chose the biggest (the sagitta) for ageing because it contains clearly visible ring patterns that are spaced wide enough apart to discriminate with a standard, scientific compound microscope.  Rings were counted from about 90 juvenile sablefish otoliths collected from offshore Alaska waters during 1995-97.  Comparing the ring  counts with the size of fish at capture revealed that Alaska sablefish grow more than 1 mm per day during their first year of life.  These growth rates are similar to those found for juvenile sablefish off Washington and Oregon.  Analyses also revealed that the first visible otolith ring (possibly the hatch or first-feeding date) formed sometime during April and May of each year (Figure 3).  The estimated hatch dates for Alaska sablefish are almost 1 month later than hatch dates estimated by other scientists for sablefish found off Oregon and Washington.  Knowledge of hatch dates is important for determining spawning time.

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Figure 4. Two visible chemical markers on a sectioned otolith
separated by about 10 micrometers.

An important component of daily growth research is to validate that the rings counted do, in fact, form on a daily basis. About twelve young-of-the-year sablefish were captured live and returned to the ABL during September 1998.  The otoliths of the live fish were marked with a chemical agent twice during captivity. The fish were later sacrificed and the number of rings between chemical markers on each otolith was compared to the number of days between marking events (Figure 4 above).   Unfortunately, the otolith rings were spaced too closely together to accurately count.  One reason for these results may be that the water used in the live tank was pumped from the bottom of Auke Bay and was colder than the surface water that young-of-the-year sablefish normally inhabit.  Typically, fish growth is slower at lower water temperatures.  We plan to evaluate the experimental procedure and then repeat the experiment.  Other goals are to back-calculate daily growth rates from otolith ring patterns and then compare growth rates with young-of-the-year abundance estimates and environmental factors such as ocean temperatures.  Such comparisons can lead to a better understanding of Alaska sablefish early life history and possibly aid in predicting the strength of future year classes.

Electronic tagging of adult sablefish

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Figure 5. Picture of electronic tag implanted in sablefish.

Auke Bay Laboratory scientists began tagging adult sablefish with electronic tags in 1998 in an effort to learn more about sablefish behavior and the marine environmental conditions in which they live.  The electronic tag records depth and temperature (Figure 5 above). Recovery of the electronic tags will help us learn about the daily, seasonal, and age-related depth movements of sablefish and increase our understanding of what part of the population is susceptible to the fishery and how this susceptibility changes during the life of the fish.  This information will help us improve sustainable harvest recommendations.

During the 1998 longline survey of the Aleutian Islands region and Gulf of Alaska, electronic tags, measuring 3/4 inches diameter by 2 1/4 inches long, were surgically implanted in the abdominal cavity of 195 sablefish. The fish also were externally marked with a flourescent pink and green tag. More electronic tagging is planned for the 2000 longline sablefish survey.

Eight electronic tags have been recovered so far, one during 1998 and seven during 1999.  One tagged  fish was at large 2 months, the other seven fish about 1 year.  Most fish were recovered near their release location, except for one fish which traveled from near Dutch Harbor to near Seward.  The fish generally were in shallower waters during summer and deeper waters during winter. Average weekly depth for each recovered fish ranged from 200 to 500 m during summer and from 400 to 700 m during winter. Average weekly temperature ranged from 3.5C  to 6.5C, but no seasonal pattern was apparent.  Individual fish traveled a wide depth range, sometimes during 1-2 days.  For example, one fish rose from about 1,100 m to 670 m in 1-1/2 days and to 220 m in 9 days during October 1998 (Figure 6).

In May 1998, “dummy” electronic tags were implanted in sablefish held in a live tank at the ABL so staff could practice surgical procedures and determine post-tagging survival.  A Juneau medical doctor  taught surgical procedures to ABL scientists. Eight tags were surgically implanted.  The tagged sablefish initially would not eat, but feeding resumed 7-10 days after tagging. One fish accidentally died during tank cleaning; the remaining seven fish survived as of September 1999, and their surgical wounds had healed.

Members of the fishing industry can help in this research and earn a $500 reward by recovering the electronic tags found inside sablefish. Tagged fish can be identified by a 3-inch- long, flourescent green and pink, external tag located near the first dorsal fin of the fish.  Text on the tag reads “Reward for Depth Sensor Inside Fish.”  Anyone who encounters fish with these green and pink tags should cut the fish open and remove the electronic tag from the abdominal cavity.  Both the external and electronic tags should be retained.  Individuals may report a recovered tag by calling collect (907) 789-6037, or mailing it to Michael Sigler, National Marine Fisheries Service, Auke Bay Laboratory, 11305 Glacier Hwy, Juneau, Alaska  99801.

Tag reporting rates

In addition to the electronic tagging study, the ABL also has conducted large-scale tag-and-release studies since 1978 to determine sablefish movement patterns. Several thousand sablefish have been tagged each year during the longline surveys. The results show that sablefish are a highly migratory species for at least part of their life, and their movement rates are great enough to affect the amount of fish available for harvest in an area. Although the results of the longline survey are the primary data used to determine sablefish quotas, tagging data from ABL’s tag and release efforts provide complementary information that has been used to augment the information from the longline survey. For example, movement rates determined from tagging data have been used to evaluate methods for allocating quota by area. The evaluation indicated that the current method of basing quotas by area on the geographic distribution of biomass is reasonable. In addition, tagging data have been used to validate ageing and to determine age of recruitment to the fishery.  Unfortunately not all tagged fish recovered by fishermen are reported to NMFS. Therefore, determination of a reporting rate is necessary to better analyze the tagging data and is an important part of the assessment.  The tag reporting rate is the  proportion of recovered tags which are actually returned.

We calculated estimates of tag reporting rates for sablefish in the commercial fishery in Alaska from 1980 to 1998.  These estimates were obtained by comparing tag returns from the commercial fishery to tag returns from the sablefish longline survey.  Sablefish catch is closely observed during the survey, so we assumed that all tagged sablefish caught during the survey are reported (i.e., tag reporting rate on the survey is 100%). A total of 11,090 commercially-recovered tags had sufficient catch information to be included in the analysis.  Tags recovered in inside waters of southeastern Alaska such as Chatham or Clarence Straits were not included because there were no survey stations in these areas. Survey-recovered tags totaled 844 for the same period.

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Figure 7.  Estimates of the tag reporting rates (fraction reported) for sablefish fishery in Alaska from 1980 to 1998.

Tag reporting rates increased from a low of 10% to 25%  from 1980 to 1982 to a peak of 47% in 1985, before dropping back to 15% in 1987 (Figure 7 above).  From that low point, the reporting rate has gradually increased and has fluctuated between 35% and 45% since 1995.  Over all years combined, the tag reporting rate has been 27%. Factors which may have influenced the tag reporting rate at various times are the number of tags available for recovery, the length of the commercial season, the presence of observers to collect the tags, and the tag reward program.

Approximately 200,000 sablefish have been tagged and released into the outside waters of Alaska from 1978 to the present during annual longline surveys.  Over half  (103,466) were released in the 5 years from 1979 through 1983.  The relatively high number of tags available during those years may have increased the awareness of members of the fishing industry and resulted in the higher tag reporting rates between 1983 and 1985.

Until 1984, the fishing season for sablefish in the Gulf of Alaska was year-round.  However in subsequent years, season length decreased as fishing effort increased.  The season shortened to 7.6 months in 1984 and to 3 months in 1985.  From 1986 through 1994, season length ranged from 1.8 months to a low of 0.3 months (1994).  Season length was restored to 8 months with the implementation of the IFQ system in 1995.  The short derby fisheries of the late 1980s and early 1990s probably had a detrimental effect on tag reporting rates.  During those fisheries, fishermen were under pressure to haul gear and deliver fish as fast as possible, increasing the chance that a tagged fish would be overlooked.  The fishing season has remained 8 months long since 1995, and tag reporting rates have remained higher than at any time other than the peak year of 1985.

Prior to 1987 the ABL sent fishermen information about the tag and release program and a small cash reward for recovered tags. Beginning in 1987, we offered a baseball cap as an alternative to cash.  In 1992, we also instituted an annual drawing for cash prizes of $1,000, $500, and two cash prizes of $250 as further encouragement to return tags.  These  incentives and publicity may be partly responsible for the gradual increase in reporting rates since 1987.

Although the increase in reporting rates is encouraging, we are still concerned that less than half the tags recovered by the commercial fishery are reported.  Given the importance of the tagging data for assessing sablefish in Alaska, we encourage members of the fishing industry to make every effort to return tags. Returning the 3-inch-long, yellow tags, along with capture location, depth, and date helps in the research, earns a cap, and earns a chance at a cash prize of up to $1,000. Recovered tags should be mailed to Nancy Maloney, National Marine Fisheries Service, Auke Bay Laboratory, 11305 Glacier Hwy, Juneau, Alaska  99801.


Sablefish research is a team effort. Besides the people listed as authors, this team includes Delsa Anderl , Jeff Fujioka, Larry Haaga, John Karinen, Sandra Lowe, Marty Nelson , Linda Shaw, Mei-Sun Yang and the longline survey vessel captains and crews of the fishing vessels Ocean Prowler and Alaskan Leader, especially Captain Jerry Kennedy Sr. of the Ocean Prowler. Thanks also to Dave Clausen, John Karinen, and Phil Rigby for reviewing this report.