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Status of Stocks & Multispecies Assessment Program

Tagging Pacific Cod Caught by Longlining: A Feasibility Cruise

Pacific cod being gaffed aboard
A Pacific cod being gaffed aboard the longliner F/V Alaska Mist.  Perspective is looking aft at the hauling station on starboard side from one deck up.

A research cruise was conducted by the Status of Stocks and Multispecies Assessment (SSMA) Programís Fisheries Interaction Team (FIT) in fall 2008 to determine the best methods for handling longline caught Pacific cod in mark-recapture studies.

The cruise was conducted aboard a vessel of opportunity, the fishing vessel (F/V) Alaska Mist, made possible by cooperation with members of the fishing industry and utilizing Cooperative Research Funds. Resulting data permitted confirmation of feasibility and refinement of plans for an extensive mark-recapture experiment scheduled for fall and winter 2009 and 2010.

Movement of Pacific cod has been observed qualitatively in the course of several studies conducted by the AFSC and the ADF&G. As stock assessments have become more sophisticated, there has been a growing need for better understanding of the movements of cod. To that end, the FIT mark-recapture experiment in fall/winter 2009 and 2010 is designed to estimate movement rates of Pacific cod among strata in the eastern Bering Sea.

The study work will be cofunded by the North Pacific Research Board (NPRB) and the AFSC. Two research cruises are planned aboard a factory longliner, with a portion of the catch tagged and released and the remainder of the catch serving as compensation to the chartered vessel.

  shelter deck of the F/V Alaska Mist
Shelter deck of the F/V Alaska Mist, looking aft.  Live tanks are on port side, just forward of the wheelhouse and quarters.  The hauling station is directly to starboard of the tanks and one deck down.  The weather shown here typified one day of the trip.  In normal weather, whitecaps commonly formed in the surfaces of the tanks whenever the lids were raised for inspecting the cod held there.

Cod caught by longline have been successfully tagged and released elsewhere in the world, but because FIT lacks expertise in this particular fishing method, a feasibility cruise was necessary to determine the best methods of removing fish from the longline, develop preliminary estimates of acute tagging mortality associated with fish removal methods, estimate expected sample sizes, and acquire expertise for collecting and interpreting longline CPUE data.

From 16 September through 8 October 2008, FIT scientist Peter Munro accompanied the Alaska Mist on a commercial longlining trip to catch Pacific cod in the eastern Bering Sea. The Alaska Mist is a 174-foot factory longliner, running an automatic longline system at 42,000-60,000 hooks per day, and processing the catch into headed and gutted frozen blocks. Two live tanks were installed on the vessel and the vessel provided untreated seawater.

The cruise ranged widely, from east of St. George Island to 100 nmi to the west of Zhemchug Canyon. The majority of fishing occurred in the latter region at depths of 100-160 m. The vessel generously allowed cod to be taken from the line for research, and the captain and crew worked closely with the AFSC scientist to work out the best way to get the cod from the longline.

During the first week of the cruise, the vast majority of cod died, no matter what method was used to remove them from the line. Considerable time was spent making sure chlorine did not enter the water supply to the live tanks, evaluating differences between surface and bottom temperatures, and trying to control such factors as hauling speed.

For several days we cooled the live tanks with ice made in the freezer hold and then gradually raised the temperature by slowly trickling in surface seawater. These efforts made no difference, other than to slow down the cod selection and tagging process even further. In consultation with colleagues in Norway and Scotland, we bled off the gas that inflated the body cavity consequent to embolism; however, there was no increase in survival rates.

Most of the fish could not survive the trauma due to stress while on the hook at the bottom and to gas embolism as the swim bladder ruptured on the way to the surface. A few cod did live however, and by the end of the first week of fishing, we had 19 survivors holding in the live tanks and increasing in vigor. This is a very small number, however, and boded ill for the future mark-recapture experiment in which we had hoped to be able to tag and release 100-200 cod per fishing day. One benefit of the first week of the feasibility study was determining the gentlest method of taking cod from the longline, and the crew working the roller became practiced at doing so without slowing down operations.

John Rand monitors catch coming aboard
Observer John Rand monitoring catch coming aboard at the F/V Alaska Mist hauling station.  Perspective is from wheelhouse, looking forward.

The second week of the cruise was devoted to close observation of cod as they died. Special note was made of characteristics of the small number that survived. Attention switched from how to remove the cod from the line to developing criteria for quickly identifying those likely to survive capture. It became apparent that selected cod died very quickly and, for the sake of observing the process, larger numbers of cod were selected from the longline, necessarily at a quicker selection rate.

Eventually two critical observations were made. First, cod had been selected for observation in batches during the first week but were selected in a stream of individuals during the second week. Second, those that survived, for the most part, could be identified within 30-60 seconds of entering the live tank.

Observation in batches placed severe limitation on the total number of cod that could be examined by a single scientist. Once this was recognized, the apparent low survival rate posed fewer difficulties if a large enough number of cod could be examined. The short time required to identify survivors combined with the constant streaming of cod aboard by the longline meant that a large number of cod could indeed be examined for tag and release candidacy.

The third week of the cruise was spent collecting data under the new survivor identification criteria and estimating the probability of surviving capture for 24 hours or more. The probability of surviving for an hour or more following capture (initial survival) was 0.18. The probability of surviving 24 hours following selection for tag and release candidacy was 0.78 or greater (this is a conservative estimate since some data were included that were intended for estimating the initial survival rate).

No mortality occurred for any cod that survived beyond 24 hours. In fact, the population in one live tank grew to 25 and remained at that level for 4 days. During that time we expected crowding and longer exposure to the presumably adverse conditions of the tank to reduce health and cause mortalities. Instead, the cod appeared to increase in vigor as they recovered from the barotrauma of capture.

These results lead us to expect to tag 100 or more cod per fishing day during the large-scale experiment scheduled for fall and winter of 2009 and 2010. This expectation is based on two assumptions: 1) that the observed initial survival rate holds and 2) that we can examine one fish every 60 seconds over a period of 10 hours of retrieval.

This examination rate is conservative, both in the period of haul back and in the number of fish that can be examined. Furthermore, the large number of cod that can be safely held in a live tank will permit large sample sizes in estimating acute tagging mortality in the course of the mark-recapture experiment. The design for that experiment includes holding tagged cod in live tanks for 24, 48, and 72 hours and observing the mortality rate directly.

By Peter Munro


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