The purse seine fishing method
Purse seine fishing employs large nets up to 2000m long and 200m deep, depending on the species of schooling pelagic fish targeted for capture, usually tuna, sardines or mackerel. The nets surround the fish and are drawn together, enclosing the fish and preventing their escape.
Yet purse seine fishing is a non-selective method of capture, and purse seine nets catch more than just tuna. Sea turtles, sharks and marine mammals, including humpback whales and bottlenose dolphins are among the non-targeted species caught in purse seine nets (see Bycatch & Incidental Capture of Cetaceans under Impacts).
It is estimated at least 6 million dolphins have been killed in the Eastern Tropical Pacific purse seine tuna fishery since the late 1950’s. The tuna fishing industry has undergone major legislative and operational changes since the 1980’s and, as a result, dolphin deaths by purse seine fishing have declined significantly.
Dolphin bycatch has been drastically reduced by more than 99%, yet around 1000 dolphins are still killed every year. Current laws prohibit the intentional setting of purse seine nets on dolphin pods, yet it is legal for fishermen to catch dolphins with the tuna and then release them. Dolphins are still being entangled, injured, stressed or crushed.
Dolphin mother-calf separation
Consuming canned tuna fished by purse seine may be indirectly contributing to the death of dolphins and their calves.
Dolphin calves stay close to their mothers’ side when young, not because they are strong swimmers, but because their proximity reduces the forces they would need to generate themselves to be able to swim independently.
Mother dolphin does the work and baby gets a free ride, with no physical contact.
This process is called drafting, and it happens due to a combination of hydrodynamics and a scientific principle known as the Bernoulli Effect. The movement of water around a mother-calf pair pulls the calf towards its mother’s side (Weihs 2004).
Tuna were originally caught using the pole-and-line technique, but purse seine nets replaced this traditional fishing method. Purse seine nets were ‘set’ on dolphin pods because tuna tend to school beneath dolphins.
‘Mother-calf separation’ is an unintended consequence of fishing Thunnus albacares (Yellowfin Tuna) by purse seine. During the chase and capture process characteristic of the purse seine technique, mother and calf pairs become separated and the calf may be lost. Most lactating dolphin mothers killed in purse seine nets are without a calf.
The frightening and stressful speed and terror of being chased by a fishing vessel severs the energetic connection between mother and calf. A calf has little chance of survival once separated from its mother.
Canned tuna and ‘dolphin-safe’ labelling
The ‘dolphin-safe’ canned tuna label is not a perfect system. Canned tuna will never be truly ‘dolphin-safe’. The label does not mean dolphin deaths have been eliminated, only that they have been reduced. Dolphin bycatch can never be zero, simply due to the unique ecological relationship that exists between tuna and dolphins.
Where the dolphins are, will be the tuna…and vice versa – although the exact nature of the ecological dynamics remains unknown. International limits exist on the maximum number of dolphins permitted to be killed in purse-seine nets, but dolphins still die for canned tuna. An alternative is tuna fished by the pole-and-line fishing method (see Pole & Line Fishing under Solutions).
‘Dolphin-safe’ labelling doesn’t account for other marine creatures killed by tuna fishing. Besides purse seine nets, commercial tuna fisheries utilize fish aggregation devices (FADs), and anchored or free-floating drift nets to attract tuna, but also other marine species, including sharks and sea turtles.
© 2016 – 2021 Seafood Free September
Weihs, D. The hydrodynamics of dolphin drafting. J Biol 3, 8 (2004). https://doi.org/10.1186/jbiol2