Fish radio

Halibut slow growth connected to fishing

March 22, 2016

This is Fish Radio. I’m Laine Welch – Halibut are small for their age and fishing is part of the reason. More after this –    conceptualdiagram

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Pacific halibut that weighed 120 pounds on average about 30 years ago tip the scales at under 45 pounds today. It’s especially true for fish in the Western and Central Gulf. Researchers point to competition for food and a changing environment as reasons for the smaller fish – now for the first time they are investigating impacts of fishing.

We found that fishing can explain between 30 and 100 percent of the observed declines in size at age in the GOA depending on which area you’re looking at.  

Jane Sullivan is a UAF graduate student based at NOAA’s Auke Bay Lab in Juneau. She, along with  NOAA and Halibut Commission scientists are gauging the effects of fishing on halibut growth.

 We took all the information that we knew about the halibut population in the 1980s when fish were big and used the computer model to fish this population at different harvest levels to see how fishing affects size at age. The general idea here is that the larger, faster growing halibut are likely to get caught because they reach the size limit at a younger age. And we found that resulting declines in size at age become greater with age because fishing affects compound with each year of fishing.  

The team modeled several scenarios, including reducing the 32 inch minimum size in the fishery and releasing halibut over 60 inches. Neither appeared to make any difference in fishing impacts on the fish size at age.

I was actually surprised to find out that implementing an upper size limit would not have any impact on size at age.  I expected that potentially protecting these fish once they reached a certain size would help alleviate the problem. But our results suggested it didn’t make any impact at all. And instead, implementing an upper size limit would  potentially cause an increase in wastage or in the fish that die after being released. The reason for that is a certain percentage  of the fish that are caught and released because of size limit ultimately die.

Sullivan said the research also found that bycatch of halibut in other fisheries is not a key factor in the size at age issue.

The majority of halibut caught as bycatch in a lot of these other fisheries are much smaller sized halibut so we don’t think there would be the same selective fishing going on as there is in the commercial halibut fishery.  

In terms of  potential changes to the fishery to protect the slow growing fish, the science points to an  unpopular solution.

The only management action that appears to make any difference is to reduce fishing effort or harvest. So by reducing effort you reduce the selective harvest of large halibut.  

Sullivan will present her halibut size at age study April 1 at ComFish.

Fish Radio is also brought to you by Ocean Beauty Seafoods, an Alaska corporation proudly supporting Alaska’s coastal communities and the Alaskans who depend on fishing for their livelihoods and culture.    In Kodiak, I’m Laine Welch.