Why do seabirds flock behind trawlers?
The answer is that fishing trawlers process their catch on board, discarding fish heads and guts overboard. They must process their catch as soon as possible because hake in particular are prone to deteriorate rapidly after they are caught, owing to a process known as “autolysis”. It is only by removing the fish guts that the freshness and quality of the fish meat may be preserved.
Although offal has no economic value whatsoever, it is highly appreciated by, and beneficial to, seabirds leading to frenzied competition between individuals and different species of birds that flock in from far and wide to take advantage of a “free” meal. (Hake livers in particular, seem to be a highly prized element in the diets of seabirds.)
The problem is that in this feeding frenzy, albatrosses (with their immense wingspans) and smaller seabirds would fly into the trawl warps as the net was being lowered to the seabed. The popular conception is that the birds were pulled under as the warps payed-out, and then drowned. Although the feeding frenzy reaches its peak when the trawl winches are active, this is not the whole picture. Observations from the New Zealand hoki fishery reveal the majority of mortalities and injuries occur at the wire/water interface and are attributable to the unpredictable movement between more or less static, but acute, cables and heaving seas, complicated by the pitching and rolling of the vessel itself.
Such sea conditions prevail for 95% of the hake fishing cycle and, without tori lines, many birds would be injured or drowned. Albatross and their near relatives, petrels, are very susceptible to impact injuries. Such strikes can be a virtual death sentence. Whereas smaller birds feeding at the interface are often taken under by the wires, they usually resurface to continue their activity without apparent harm; larger birds don’t get off quite so lightly from a dunking.
Variable sea surface conditions compound the seabird trawling problem in another way. Sea conditions worsen markedly in the Southern Hemisphere winter, when the seabird assemblages around fishing trawlers are four to five times more intense. Storm conditions and seabird congestion combine to make seabird mitigation activities – such as the consistent and correct deployment of tori lines – absolutely critical at the very time when fishing is at its most difficult and the conditions are at their worst.