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Big Fish Matter

Aug 03,2000 Leon Roskilly


I’ve seen quite a few Powerpoint slides recently, all purporting to show the factors that will govern the size of a particular stock.

Simple really; size of stock, rate of recruitment, availability of prey, predation, fishing effort etc.  Change a value for any factor and the model will churn out a certain stock size.

So, why does it not always happen like that?

Why do some stocks refuse to recover?

Well recent research has provided part of the answer, and it’s all to do with the size of fish.

Older, more mature, bigger fish carry a lot more spawn than do early spawners.

And not only do they produce more eggs, but the eggs and fry of these older bigger fish seem to be far more resilient than the offspring of younger fish.

So a stock with plenty of big fish still in the population is able to replenish itself more effectively than the same spawning biomass of predominately young spawners.

And yet our fisheries conservation measures are all aimed at preserving the young stock, the future spawners, and harvesting the bigger fish.

And that has another effect. 

Again research has shown that, in as few as four or five generations, a genetic shift takes place, favouring the survival of smaller individuals whose offspring are also likely to be smaller, and being smaller will produce fewer eggs than their larger ancestors.

And in continuously removing sizeable specimens (which happens when many minimum landing sizes are set below the size of first spawning) what is know as a ‘recruitment fishery’ develops.

Most fish of spawning and pre-spawning age are efficiently removed, and relatively few individuals produce sufficient eggs to provide a new generation to be fished.  Such fisheries are described as ‘sustainable’.

But what is alarming many researchers is that the reliance on a relatively few individuals to produce future stocks considerably reduces the genetic diversity of the stock, and increases that lack of diversity with each succeeding generation.

Why does that matter?

‘The balance of nature’ is a myth.

Things are changing all of the time, sometimes slowly, sometimes quickly.

That faces any species with a challenge, ‘Adapt or Die’

It is a species’ genetic diversity that determines its ability to adapt to environmental challenges.

And right now, environmental challenges abound for our marine species like almost no other time in history.

And for some species, the females are considerably bigger than the males, so removing the larger specimens unbalances the natural gender balance.  There may seem to be plenty of fish, especially smaller fish expected to grow on. But when most of the apparent stock is male, recruitment may already be in deep trouble.

So, it would seem that most of our fisheries management is inadvertently directed at

  • Removing the best spawners, with the most viable offspring
  • Driving the average size of fish down with smaller fish producing less spawn
  • Destroying the genetic diversity of species, making them extremely vulnerable to new environmental challenges
  • Hiding the fact that the stock of females, capable of spawning future generations, has already crashed.

 That hardly seems to be a fisheries management policy with a future.

So what can we do?

That is a question more easily asked than answered. 

Obviously, we must find ways of preserving both the smaller fish as we do now, but also the larger fish that are needed to generate a healthy and resilient population.

Where species are managed for recreational purposes, and catch and release is a viable option, in many cases ‘slot limits’ have replaced minimum landing sizes. Both the smaller and the larger fish are released, and anglers allowed to consider taking the middle-sized fish for their own consumption.

In some cases, selective gear can be used i.e. gill-net mesh sizes allow the smaller fish to pass through, the larger fish bounce off, and only the targeted size becomes enmeshed.

Rod and line commercial fisheries are also candidates for slot limits (but difficult to sell if other commercial fisheries are allowed to take any size).

In some cases, fish will shoal by year group, so shoals of small and bigger fish can be avoided and only shoals of ‘takeable’ size targeted.

Areas where larger fish congregate, i.e. for spawning, can be made into ‘No Take Zones’ at certain times of the year.  In considering where to place Marine Protected areas, the protection of the larger sized fish should become a high priority.

But it is in the fisheries where trawls are used to take many tonnes of fish, particularly where fish of different species and sizes of fish spawn together that we need to find innovative solutions.

Whatever the problems, if a solution is to be found to our fisheries crises, we must find ways to protect the bigger specimens, and we must adopt policies that will preserve the larger specimens allowing them to get on with the job, not only of providing future offspring, but of ensuring the future viability of their species.




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