By Atuna
Among several environmental issues concerning tuna stocks worldwide, skipjack tuna so far claimed abundant always managed to bring some comfort for canned tuna lovers.
However, there has been a considerable drop of skipjack catches in the Eastern Pacific Ocean (EPO) during the past two years, pulling prices up and causing turmoil among companies which rely on tuna fishing in the area to find enough supply.
Fishermen have alleged that the El Nino could be causing the lower catches, but the Inter-American Tropical Tuna Commission (IATTC) -the RFMO managing the area- did not confirm that hypothesis, neither gave an answer to the puzzle…until last week.
IATTC stock assessment scientist, Mark Maunder, has recently released an updated document addressing all the issues and uncertainties when assessing skipjack tuna in the EPO. The objective of the document is to motivate the elaboration of new methodologies to address current gaps: “Since the stock assessments and reference points for skipjack in the EPO are so uncertain, developing alternative methods to assess and manage the species that are robust to these uncertainties would be beneficialâ€, stated Maunder.
Some of the assertions in the document can be alarming such as “(…) neither the biomass- nor fishing mortality-based reference points, nor the indicators to which they are compared, are available for skipjack in the EPO. (…). Currently, there is no reliable index of relative abundance for skipjack in the EPO†he continued.
Maunder explores the possibility of a hidden biomass of large skipjack that cannot be estimated, considering previous assessments. He believes that one of the major problems mentioned in the document is the uncertainty as to whether the catch per unit of effort (CPUE) of the purse-seine fisheries is an appropriate index of abundance for skipjack, particularly when the fish are associated with fish-aggregating devices (FADs).
The CPUE is a measure in the current assessment as the amount of searching time required to find a school of fish on which to set the purse seine, and this is approximated by number of days fished. However, it is well known that skipjack fishing is done mostly by purse-seiners on unassociated schools, using FADs – a real time saver. This can dramatically change the data results on the matter.
According to Maunder, a full management strategy evaluation (MSE) for skipjack would be the most comprehensive method to develop management strategies, however, the priority is now to develop MSE for bigeye and yellowfin tuna, species which assessment had confirmed overfishing.
The lack of a definitive answer explaining lower skipjack catches in the EPO, may be a better answer after all. IATTC’s main concern with the skipjack stock is the constantly increasing exploitation rate although data and indicators have yet to detect any adverse consequence of this increase.
The average weight of the fish is near its lower reference level, which can be a consequence of overexploitation, but it can also be caused by recent recruitments being greater than past recruitments. It may take a while to know which one is the real cause for this and many other dualistic skipjack stock issues. How tuna stakeholders manage the species until then could influence the verdict.
