Catching those Tuna: The challenges of purse seine versus longline tuna fishing

cropped-yellow-fin-tuna-school3.jpg

Today I was reading an older but still relevant highly relevant editorial in FIS news that got me thinking about the two major tuna fishing methods: tuna long lining and tuna purse seining in a totally different way.

The editorial article the challenges of purse seine versus longline tuna fishing (FIS 10 May 2012) looks at the differences between the two methods from the point of view of product destination.

This is a novel approach, as in my experience, the distinction is usually made (by eNGOs) with reference to performance of the gear type against the marine environment by highlighting by-catch rates and species types likely to be caught in addition to the target tuna species. For example contrast between the two gear types has often be demonstrated by captures of dolphins in purse seines or captures of turtles on longlines. Recently this contrast has been clouded with the plight of pelagic sharks being the new causes célèbre by global  eNGOs, and sharks feature in all harvest methods to varying degrees.

The article demonstrates the importance of both major methods (longlining and purse seining); not only because they target completely different fish, but because they have different socio-economic functions. This approach contrasts heavily with the eNGO ‘effects of the fishing gear’ approach which concentrates almost exclusively on the negative aspects of long line and purse seine gear deployment. A target based approach (as used this article) has a direct application to managing the sustainability of the target species.

The article separates the market:

“The tuna industry can be split into a sub-industry of high-priced fresh fish market (particularly of sashimi) mostly supported by longliners and one of low-priced canned tuna mainly supplied by purse seiners.”

“As the demand for canned tuna has been expanding very rapidly, the purse seine fleet has also expanded rapidly, according to Dr Peter Miyake of the Organisation for the Promotion of Responsible Tuna Fisheries (OPRT)”.

Dr Peter Miyake is a leading tuna researcher who has worked for many international tuna fisheries management groups, including the Inter American Tropical Tuna Commission (IATTC) and the International Commission for the Conservation of Atlantic Tunas (ICCAT). He now participates in the scientific meetings as a visiting researcher at National Research Institute of Far Seas Fisheries of Japan.

Miyake explains that purse seine fishing is is the most economic fishing method; arguing that the deployment of the fish aggregating devices (FAD) saves considerable fuel.

“But if companies were to catch all tuna only using purse seiners, the total catch which can be sustainable would be much lower, and without a longline fishery, there would be no tuna available for sashimi consumption and the reduction in total economic yield would be considerable”.

An important difference between the two harvest methods can be seen in the composition of the catch.

“Purse seiners catch abundant relatively inexpensive young small-sized tuna, while longliners catch much less of very high-quality expensive large-sized fish.”

Miyake points out that:

“A cohort [statistician speak for fish that are from the same place and same year] of tuna gains mass until a certain size/age and thereafter mass will decline (i.e. natural mortality loss is greater than growth gain). This critical point is about 40 kg in yellowfin tuna and 70 kg in bigeye tuna, which correspond to captures by longliners.”

“Therefore, the total weight of fish which can be sustainably harvested by purse seiners alone would be much less than those caught only by longliners. In fact, the current maximum sustainable yield (MSY) of bigeye tuna taken by longline and a greater number of purse seine fisheries in the Pacific Ocean has been almost halved compared to the level 20 years ago when bigeye tuna were caught by longline alone.”

TUNA CATCH IN METRIC TONNES IN THE WCPO.Tuna have been caught for centuries in the Pacific Islands.  This was usually from canoes and often using handlines, troll gear, or pearl shell lures. Starting in the early 1900s larger scale tuna fishing gear was introduced into the region. Today four types of gear produces the vast majority of the tuna catch in the Pacific Islands region:  purse seine, longline, pole-and-line and trolling (see http://www.wcpfc.int/statistical-bulletins)

TUNA CATCH IN METRIC TONNES IN THE WCPO.
Tuna have been caught for centuries in the Pacific Islands. This was usually from canoes and often using handlines, troll gear, or pearl shell lures. Starting in the early 1900s larger scale tuna fishing gear was introduced into the region. Today four types of gear produces the vast majority of the tuna catch in the Pacific Islands region: purse seine, longline, pole-and-line and trolling (see http://www.wcpfc.int/statistical-bulletins)

Miyake points that harvesting tunas (e.g. big eye, yellowfin and skipjack) by longline alone would more than likely increase their maximum sustainable yield (MSY) [MSY is the sustainable yield of natural capital that can be extracted without reducing the base of the natural capital itself]. But that such a measure would have profound socio-economic effects on the tuna canning industry.

As you know I am a huge advocate of the venn diagrammatic (Rio 20+) definition of sustainability [where sustainable use is based on three equally considered environmental, economic and social components]. Where it follows that any sustainability decision based on one component at the expense of the others is destined to fail.

Therefore the social impact of such a sustainability decision would potentially devastate the global canning industry due to the severe shortage in tuna and an exaggerated price of the line court fish that are available. This state of affairs would undoubtedly have an economic response. The exaggerated prices and shortage of canning quality fish would initiate a boom, which would result in a speedy substantial increase in longline vessel capacity.

Big Eye PNA

Bigeye tuna (Thunnus obesus). Source: Big Tuna Business Interests Clash With PNA Goals (see http://pna.atuna.com/)

This is happening now. 

The Taiwanese are exploiting the lacuna in the market created by the market’s disdain for large mechanical longline vessels and the use of FADs  by purse seiners [in addition to the market’s better treatment of fresh sashimi species] and are deploying thousands of small highly mobile vessels, that with snap freezing ability, and by virtue of the american longline technology they use, are able to land catches of up to 120+ t per annum.

In an article in published in the SPC Fisheries Newsletter (January/April 2012 #137) ‘OPRT study echoes PITIA concerns over rapid increase in small tuna longliners’. This article provides the results of a study by Japan’s National Resource Institute of Far Seas Fisheries which were presented at OPRT’s fourth annual seminar in Tokyo on 10 February 2012 by Jiro Suzuki. Suzuki argues that the real status of the small-scale longline fishing industry is currently not well understood by regional fisheries management organisations. Suzuki estimated that:

“Up to 5,400 small longline vessels could be operating globally, with 1,800 of these operating within the waters of the Western and Central Pacific Fisheries Commission. An additional 50–60 Taiwanese small longline vessels with onboard super-refrigeration status are reportedly currently under construction in Taiwan.”

Furthermore and more sobering, according to Mr Suzuki, is the impact of the burgeoning of small-scale longline vessels raises concerns about the sustainability of bigeye and yellowfin stocks [and as I have heard from New Zealand sources skipjack stocks as well].

Suzuki maintains that the sudden increase in Taiwanese small, mobile long line capacity stems from:

  • The exploitation ‘indigenous aspirational development’ clauses within WCPFC agreements. With the use of economic aid, Taiwan is able to secure the accommodation of additional fishing in various Pacific Island countries (PICs), in conjunction with the PICs growing drive to develop their tuna fishing industries;
  • Former Taiwanese shark-finning vessels are converting to albacore vessels due to prohibitions introduced on sharkfin fishing which has adversely affected the profitability of these vessels targetting sharks for their fins;
  • Vessel construction and operation of small- and medium-sized longliners is far more economical than operating large-scale longliners, which by comparison are in decline; and
  • traditional refrigerated carriers are being gradually replaced with more versatile individual super refrigerated (-60°C) cargo containers, which are well suited to accommodating small catch consignments; super-refrigerated storage capacity has also been developed on vessels.

Given the economic and social ramifications of purely biological based potential management options [Dr Peter Miyake] concluded that:

“The best alternative is to seek a point of compromise through a fair balance of social, economic, environmental and biological factors. Scientists must come up with an unbiased and transparent way to achieve a balance among these various factors, without being affected by prejudiced propaganda, money or political pressures.”

I must say I do agree.

Menhaden fishing - purse seine boats encirclin...

HOW ARE TUNA HARVESTED?

What is a purse seine? What is  longlining? Are there any other methods used to take tuna?

Purse seine

longline

Pole and Line

tuna troll

These images are from the Secretariat of the Pacific Community (SPC) website. For more on tuna harvesting methods see www.spc.int/OceanFish/en/tuna-fisheries/fishing-methods.

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6 thoughts on “Catching those Tuna: The challenges of purse seine versus longline tuna fishing

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  6. Sono stato molto contento di aver trovato questo sito. Voglio dire grazie per il vostro tempo per questa lettura meravigliosa! Io sicuramente mi sto godendo ogni post e ho gi salvato il sito tra i segnalibri per non perdermi nulla!

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