Most Tuna Stocks Might Be Less Migratory Than Often Assumed ff

Written by Natalia Freitas for atuna.com

The depletion of tuna stocks is a current issue important to the scientific community and the tuna industry. Overfishing, besides being an environmental threat, has also become an economic concern because the consequence could be the disappearance of raw material for the global industry.

For nearly three decades, countries have been trying to reach a consensus on how to preserve the tuna species and consequently the Regional Fisheries Management Organizations (RFMOs) were created especially for tuna.

Year after year, the RFMOs meet with Government and industry representatives to discuss solutions to stop tuna depletion through the use of fishing quotas and tuna ban periods. However, uncertainty about the precise status of tuna stocks is often used as a reason to hold back or postpone decisions.

Unlike many would assume, the uncertainty in predicting tuna stocks doesn’t lie in the fact that tuna is a highly migratory species, but in the lack of data available.

Atuna.com went to IATTC Stock Assessment Scientis, Mark Maunder, for answers. He shares his views and findings to confirm the importance of local tuna management combined with close global scientific cooperation.

Atuna.com: The tuna stocks numbers from several research centers around the globe often differ. Which data should we believe?

Maunder: It depends a lot on what information is available for the stock being assessed. For tuna species, it’s very difficult to have a survey for such wide areas of the globe, and we don’t have regular aging information. Typically we have to rely on catch per unit effort (CPUE) data, as an index for the abundance along with length measurements to deduce the age. Relying on these catch data makes the assessments less reliable than if we had better data. The data used and reliability of the assessment also depend on what assessment model is applied, and there are many different models used nowadays.

The article Evaluating Tuna Management in the East Pacific Ocean, published in 2006 by Maunder and Shelton Harley, shows how different the results of stock assessment can be from one model to another: “For data-rich stocks, we can determine whether the objectives to maximize the catch (yield) have been achieved, but even these results have alternative interpretations. Unfortunately, when data are limited, the management objectives cannot be evaluated.” The article explains all the different interpretations in calculating the maximum sustainable yield (MSY) and defines the existing management goals as often vague, from a stock-assessment perspective. Therefore, the data itself may not be unclear, but different stock assessment models can give different readings of it.

Atuna.com: is it possible to assess tuna stocks regionally?

Maunder: That can be a problem in tuna assessment, because you may not be able to assume that you’re looking at the same stocks. In species like albacore and bluefin, which are highly migratory species, the adult fish could be on one side of the ocean and the juvenile on the other. However, we have found from recent tuna tagging information that species like yellowfin, bigeye, and skipjack do not move that much. Therefore, local depletion occurs and fishing in certain areas will indeed influence local tuna stocks.

In the promotion for next week’s Spatial Analysis for Stock Assessment 2008 Workshop, to be held between October 14^th and 17^th, the IATTC states: “It is clearly evident that stock and fishery dynamics vary regionally and this variability should be considered in the assessment and management of fish stocks. Currently, few stock assessment models directly address spatial variability and methods need to be developed to improve this situation”.

Skipjack, yellowfin and bigeye tuna represent 93% of the world tuna catches. Considering the new findings about the lower migration of those species, it could be concluded that local tuna management and the RFMOs actions are even more essential to the preservation of the local tuna stocks and industry, than we might have thought before.

Atuna.com: Should all the tuna organizations around the world use the same stock assessment model?

Maunder: In my opinion that should be done. I believe that the same method should be used in all the oceans. Even though there has been a lot of cooperation between some of the RFMO scientists (e.g. IATTC and SPC), having common stock assessment modeling approach would be beneficial for improving understanding of the assessment methods, promoting development of the modeling, and facilitating reviews.

The Spatial Analysis for Stock Assessment 2008 Workshop promoted by IATTC will have participants from many different countries and institutions. Besides discussing stock assessment models, there will be a workshop in parallel about Stock Synthesis, software that can be used in the assignment. The software can be downloaded for free in the NOAA Fisheries Toolbox page.

Atuna.com: How accurate are the tuna stock numbers today?

Maunder: It really depends on the species. We don’t have that much confidence in the assessment of skipjack tuna, since the stock size can double very rapidly. The assessment for bigeye tuna is more certain, but not as certain as for yellowfin tuna. 

Atuna.com: How important is it to look at tuna stocks at a regional level?

Maunder: Since the movement of the fish for some species is limited, it’s important to look at spatial characteristics. It’s important to integrate all the information we have (tagging and other data) to evaluate at what level management should occur. There has been a lot of work done in that line by the WCPFC (by SPC), since many of their member countries are small island nations which EEZs’ divide the management area. To invest in local stock assessments can affect the type of all the local tuna management applied.”