Is selectivity wrong?

The modern management mantra is that we should keep the nets clear of the smaller and younger fish. We’ve been told to fish only the older, larger fish and to the let smaller, younger fish grow, mature and procreate. The assumption has been that the surviving spawners will produce huge numbers of eggs and larvae, far more than needed to replenish the population. For this, scientists and managers have been prescribing minimum hook sizes, minimum mesh for gillnets, trawl codends and seine bags.

Now some fisheries scientists have started to question many of those almost dogmatic assumptions. Two Icelanders, Dr. Jonas Bjarnason, and Jon Kristjansson have argued against selective fishing. Although working from different points of view, both have reached the same conclusion: that sustained creaming off the larger and more prolific individuals brings about stock impoverishment and this is something which a reduction in quotas and even moratoria will not cure.

Their view has also been echoed in the International Council for the Exploration of the Sea newsletter (No. 40), by Dr. Mikko Heino of the Bergen Institute of Marine Research: “Does fishing cause genetic evolution in fish stocks?” Mikko Heino and Ulf Dieckmann have also written that the collapse of 12 important North Atlantic and North Sea stocks was preceded by fish maturing at lowered ages and at smaller size.

Small Grand Banks

In the April issue of ‘Nature’ last year, Norwegian fishery scientist E.L.Olsen and his co-authors analysed some 30 years of data involving the plunge in Atlantic cod populations around southern Labrador and Newfoundland’s Grand Banks. They concluded that there was a decline in the ages and sizes at maturity, even before the northern cod population had collapsed. They believe the cause was selective fishing pressure against those larger individuals that genetically tend to mature later. The resulting genetic change in the northern cod’s population now slows down its recovery, because the smaller and weaker fish cannot produce as many and as strong offspring with desirable genetic properties as could the older fish.. Early detection of such an evolutionary process could serve as an early warning system for trouble ahead.

According to Steve Berkeley, a research biologist at the University of California, Santa Cruz campus, the same phenomenon is particularly striking in long-lived rockfish species. ‘Something is just not right with how we are disproportionately removing older and larger fish.’ — he reported to a recent meeting of the American Association for the Advancement of Science

” Sorry – we got it a bit wrong all those years ago. We shouldn’t be selectively protecting just the young ones, but adding special protection for the biggest fish. Our research shows that you need to maintain older fish in the population, because those are the most successful at reproducing. But normal fishing at what we now think of as safe levels will not maintain old fish in the population,” Berkeley said.

Genetic selection

Iceland’s Dr.Bjarnason explains that due to selective fishing, more and more stocks abound with smaller, slower growing fish with inferior reproductive qualities. A genetic change occurs in fish populations and subsequent age groups mature sooner, spending most of their energy on sexual development and spawning at the expense of body growth. They are more vulnerable to predation and their natural mortality increases. It is quite obvious that culling the older breeders that produce more and stronger larvae can cause genetic selection within fish populations. In almost all cases of stock collapse in the North Atlantic these symptoms have been present, but downplayed or even ignored by ‘official’ science. These findings suggest that there are fundamentally erroneous assumptions underlying today’s decisions on the management of stocks. These mistakes could threaten the long-term supply of fish and survival of stocks. Current models assume that (1) within a fisheries management unit (stock), all offspring have an equal probability of survival and of potential productivity, (2) local depletions are temporary and replenishment will come from other areas and (3) fishing effects are temporary and reversible. Thus, most managed fisheries are dependent only on maintaining a target level of mature ‘old’ adults that had spawned at least once, while any decline below this target will be remedied by short-term restrictions on total catch.

So, how should the industry respond to this fresh perception of the issue? The existence of contemporary evolution and its effects, which can hardly be disputed, require changes in fisheries management thinking. That must mean, among other things, a change in the approach to selective fishing based on mesh and hook sizes, as well as timely reduction of excessive fishing pressure as soon as distinct genetic changes appear, rather than after populations have already begun to decline.

Older and wiser

In view of all those new (and not-so-new findings) and managerial inertia, we must ask whether there are workable alternatives to the present selective methodology. Management to date has focused on reducing fishing pressure by reducing the number of fish caught. However, to avoid removal of mainly large individuals from the stock and its consequent degeneration, we must now find a way of allowing fishermen to catch fish which have been considered as undersized and at the same time allow survival of sufficient numbers of larger fish. This has to be compensated by other and, hopefully wiser, management steps, tailored to each fishery.

Management by quota will not be effective, because the quotas would now include also smaller and cheaper fish, which would only increase discards and high-grading. The new strategy, therefore, would tend towards input-control. That could include access limitations, area and seasonal closures, days-at-sea allocation, gear-size limitations, obligatory weekends in harbour, vessel displacement and engine power and so forth. Some strong vested interests would have to be dragged kicking backwards out of their preferred quota systems, a tough but essential task if we do not want to end up with minor size cod and haddock throughout our fisheries.