Juvenile migration

The IPHC’s current understanding of juvenile Pacific halibut migration is the result of a variety of information sources including occurrence data from National Marine Fisheries Service (NMFS), IPHC fishery-independent setline surveys (FISS) and tagging projects that span several decades. A summary follows with links to more information below.  

Distribution of early life stages

Pacific halibut spawn along the continental shelf edge and embryos and larvae are carried away from the spawning grounds by prevailing currents flowing westward in the Gulf of Alaska, and north and west in the Bering Sea. Pelagic larvae depend on currents to transport them to areas of plankton production where they are able to feed, and eventually settle inshore in shallow nursery areas. Juvenile Pacific halibut begin migrating from the nursery areas within a year or two of settlement in a direction counter to the currents that transported them there, i.e. to the east and south in the Gulf of Alaska and to the south, north, and west in the Bering Sea (Fig. 1). They also disperse to greater depths as they age.

Figure 1. Conceptual model of Pacific halibut migration patterns

Pacific halibut are observed as embryos and larvae in the National Oceanic and Atmospheric Administration ichthyoplankton surveys, but are essentially invisible in their typical flatfish form to any survey gear until they become vulnerable to the NMFS groundfish trawl survey gear at 15-20 cm fork length or at 1-2 years of age. The incidence of Pacific halibut in aggregate that have been caught during the NMFS trawl surveys since the 1980s can be seen in Figures 2-6. While the largest numbers of juvenile Pacific halibut continue to be detected on or near nursery grounds (around Kodiak Island and inner Bristol Bay), it is clear that a portion of juveniles begin to disperse at ages as young as 2 years old. Three-year-old Pacific halibut are found in small numbers in the central Gulf and southern Bristol Bay, and 4-year old Pacific halibut are found throughout the Gulf of Alaska and into the Aleutian Islands. Five- and 6-year-old Pacific halibut are found in greater numbers coastwide and at greater depths throughout their known range. Bottom trawl surveys in British Columbia have observed a small number of young Pacific halibut, but those are not represented here. [Primary source: Sadorus et al. (2015)]

 

Figure 2. Distribution of age-2 halibut caught aboard the NMFS groundfish trawl surveys. Shaded squares indicate numbers caught per station.

Figure 3. Distribution of age-3 halibut caught aboard the NMFS groundfi sh trawl surveys. Shaded squares indicate numbers caught per station.

Figure 4. Distribution of age-4 halibut caught aboard the NMFS groundfi sh trawl surveys. Shaded squares indicate numbers caught per station.

Figure 5. Distribution of age-5 halibut caught aboard the NMFS groundfi sh trawl surveys. Shaded squares indicate numbers caught per station.

Figure 6. Distribution of age-6 halibut caught aboard the NMFS groundfi sh trawl surveys. Shaded squares indicate numbers caught per station.

Juvenile migration patterns

Most of the knowledge of juvenile migration patterns for Pacific halibut originates from traditional tagging (external tags) conducted between 1963 and 1986. Over 120,000 trawl-caught fish from the Bering Sea and Gulf of Alaska were tagged as part of the “Trawl Recruitment Series” (Kaimmer 2000). A formal tag-recapture analysis of migration rates for juvenile Pacific halibut resulted from the peak tagging years of 1980 and 1981 (67,000 juveniles) and estimated migration rates between IPHC Regulatory Areas 2 and 3 (Hilborn et al. 1995). Although no formal tag-recapture analysis is available for juveniles < 65 cm in length tagged in Area 4, between 20% and 30% of juveniles tagged in Area 4 were recaptured outside of the area. Raw recovery proportions for Pacific halibut from 2 to 6 years of age and for Pacific halibut < 65 cm in length at the time of tagging are listed in Tables 1 and 2, respectively. For Pacific halibut < 82 cm in length, between 50% to 60% of tag releases occurred in the proximity of the Aleutian Islands between Unimak Island and Unalaska Island, with the remaining releases being found spread along the Bering Sea edge, flats, and the Bering Sea Closed Area. Table 3 shows recovery proportions and total numbers of Pacific halibut of < 65 cm in length, when dividing the combined Area 4 into Bering Sea and Gulf of Alaska components. Area 4 migration rate estimates are available for Pacific halibut of 65 to 80 cm in length (Deriso et al. 1983) and suggest very similar emigration rates (around 23%) for IPHC Regulatory Area 4 and Area 3B, although they do not differentiate between Bering Sea and Gulf of Alaska components. [Primary source: Valero and Webster (2012)]

Table 1. Raw recovery rates of externally-tagged Pacific halibut ages 2 to 6 tagged in IPHC Regulatory Area 4 (from Clark and Hare, 1998).

Recovery Area
4	3	2C	2B	2A
0.704	0.177	0.038	0.062	0.020

Table 2. Raw recovery rates of externally-tagged Pacific halibut < 65cm in length at the time of tagging in IPHC Regulatory Area 4, summarized from IPHC database.

Recovery Area
4	3	2C	2B	2A
0.763	0.142	0.027	0.050	0.018

Table 3. Recovery proportions by release and recovery areas of externally-tagged Pacific halibut < 65 cm in length at the time of tagging. Total numbers are provided in the last column. IPHC Regulatory Area 4 is presented as separate Bering Sea (BS) and Gulf of Alaska (GOA) components. The values in bold represent the proportion that was both released and recovered in the same area.

Release Area	Recovery Area
	4 BS	4 GOA	3B	3A	2C	2B	2A	Total recovered
4 BS	0.748	0.047	0.038	0.085	0.011	0.049	0.021	527
4 GOA	0.225	0.250	0.100	0.275	0.025	0.125	-	40
3B	0.014	0.007	0.452	0.290	0.071	0.148	0.017	714
3A	0.001	-	0.020	0.699	0.063	0.192	0.025	1,625
2C	-	-	-	0.022	0.816	0.160	0.002	463
2B	-	0.000	-	0.004	0.025	0.967	0.005	3,077
2A	-	-	-	0.053	-	0.105	0.842	19
								6,465

The IPHC conducted a large-scale Passive Integrated Transponder (PIT) tagging effort in the mid-2000s. The strength of this type of tag was that it was detectable only by specialized equipment used by IPHC samplers, and thus alleviated the reporting bias inherent in external tagging. The tags were placed during the IPHC fishery-independent setline survey and did not specifically target juvenile Pacific halibut, but a component of fish caught during the surveys are < 82 cm in length and, therefore, results of the project allowed the calculated estimated probabilities of movement based on length.

For western areas, the estimated probability that a fish migrates in a given year declines with increasing length. Estimated annual emigration probabilities are very high for smaller IPHC Regulatory Area 4A fish, over 0.5 per year for Pacific halibut < 70 cm in length, and remaining over 0.2 for fish up to 95 cm in length. For Area 3B, Pacific halibut < 80 cm in length have at least a 0.15 probability of moving each year, a rate that declines with increasing length. For Areas 2B, 2C, and 3A, the relationship between emigration rates and fish length is less clear (Fig. 7). [Primary source: Webster et al. (2013)]

Figure 7. Estimated relationship between annual emigration probability and Pacific halibut length from modelling of the PIT tag data. Dashed lines are upper and lower 95% Bayesian credible interval bounds.

Migration between ocean basins

Data from Bering Sea tagging programs show that Pacific halibut < 65 cm in length have historically had high rates of movement into the Gulf of Alaska: depending on the release area, 70-90% of tagged Pacific halibut were recovered in the Gulf after eight or more years following release.  While the general pattern was for a south and eastward movement across the Gulf, there was also some migration of small Pacific halibut to the west, towards IPHC Regulatory Area 4B for all release regions. Studies tagging larger fish have found much less movement into or out of Area 4B than other regulatory areas (Valero and Webster 2012), but these data provide evidence that the population in Area 4B is connected with that in the eastern Bering Sea through the westward movement of juvenile fish.

While the raw data provide evidence that juvenile Pacific halibut tagged in the Bering Sea have, over a period of several years, high rates of migration into the Gulf of Alaska, the juvenile tagging studies were never designed with the goal of producing statistically sound estimates of migration rates. Low recovery rates for the most consistent grid design, high recovery rates resulting from unrepresentative sampling, and the lack of a consistent, contemporaneous tagging program in the Gulf of Alaska make these data unsuitable for formal statistical modelling.  The data do show that a few years after release, a majority of the recovered tagged Pacific halibut are found in the Gulf of Alaska. However, it is also likely that a greater intensity of longline fishing in the Gulf of Alaska than in the Bering Sea leads to tagged fish that move to the Gulf of Alaska to more likely be recovered than those that remain within the Bering Sea. Raw out-of-region recovery rates, therefore, imply a higher annualized movement rate than is likely to have been occurring.  Even if we overlook the shortcomings of the design, an estimate of annual movement rates cannot be derived without additional information on the relative probabilities of tag recovery in the Bering Sea and Gulf of Alaska. [Primary source: Webster (2015)]

Ongoing migration research

In 2015, the IPHC launched an external wire tagging effort targeting Pacific halibut < 82 cm in length caught during the NMFS trawl surveys which cover the Bering Sea (annually), Gulf of Alaska (biennially), and Aleutian Islands (biennially). The objective of the project is to update our understanding of juvenile migration both within and between ocean basins. In 2016, the juvenile tagging effort was expanded to include part of the IPHC fishery-independent setline survey catch, and in 2017 the project was expanded to all setline survey areas where the sampling rate for otoliths was less than 100%. Given the recent commencement of the project, there have been very few recoveries to date, but that is expected to improve over time. Tagging will continue for the next several years.

References

Clark, W. G. and Hare, S. R. 1998. Accounting for bycatch in management of the Pacific halibut fishery. N. Am. J. Fish. Mgmt. 18:809-821.

Deriso, R. B.  and Quinn II, T. J. 1983.  The Pacific halibut resource and fishery in regulatory Area 2: II Estimates of biomass, surplus production, and reproductive value.  Int. Pac. Halibut Comm. Sci. Rep. 67.

Hilborn, R., Skalski, J., Anganuzzi, A. and Hoffman, A. 1995. Movements of juvenile halibut in IPHC regulatory areas 2 and 3. Int. Pac. Halibut Comm. Tech. Rep. 31.

Kaimmer, S. M. 2000. Pacific halibut tag release programs and tag release and recovery data, 1925 through 1998. Int. Pac. Halibut Comm. Tech. Rep. 41.

Sadorus, L. L., Stewart, I. J., and Kong, T. 2015. Juvenile halibut distribution and abundance in the Bering Sea and Gulf of Alaska. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2014:367-404.

Valero, J. L. and Webster, R. A. 2012. Current understanding of Pacific halibut migration patterns. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2011: 341-380.

Webster, R. A. 2015. Trawl tag releases of small halibut in the Bering Sea. Int. Pac. Halibut Commission Report of Assessment and Research Activities 2014: 475-510.

Webster, R. A., Clark, W. G., Leaman, B. M., and Forsberg, J. E. 2013. Pacific halibut on the move: a renewed understanding of adult migration from a coastwide tagging study. Can. J. Fish. Aquat. Sci. 70: 642-653.

Additional sources of information

Best, E. A. 1969. Recruitment investigations: Trawl catch records Bering Sea, 1967. Int. Pac. Halibut Comm. Tech. Rep. 1.

Best, E. A. 1969. Recruitment investigations: Trawl catch records Gulf of Alaska, 1967. Int. Pac. Halibut Comm. Tech. Rep. 2.

Best, E. A. 1969. Recruitment investigations: Trawl catch records Eastern Bering Sea, 1968 and 1969. Int. Pac. Halibut Comm. Tech. Rep. 3.

Best, E. A. 1969. Recruitment investigations: Trawl catch records Gulf of Alaska, 1968 and 1969. Int. Pac. Halibut Comm. Tech. Rep. 5.

Best, E. A. 1970. Recruitment investigations: Trawl catch records Eastern Bering Sea, 1963, 1965,and 1966. Int. Pac. Halibut Comm. Tech. Rep. 7. 

Best, E. A. 1974. Juvenile halibut in the eastern Bering Sea: Trawl surveys, 1970-1972. Int. Pac. Halibut Comm. Tech. Rep. 11.

Best, E. A. 1974. Juvenile halibut in the Gulf of Alaska: Trawl surveys, 1970-1972. Int. Pac. Halibut Comm. Tech. Rep. 12. 

Best, E. A. 1977. Distribution and abundance of juvenile halibut in the southeastern Bering Sea. Int. Pac. Halibut Comm. Sci. Rep. 62.

Best, E. A. and Hardman, W. H. 1982. Juvenile halibut surveys, 1973-1980. Int. Pac. Halibut Comm. Tech. Rep. 20.