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September 21, 2007

Little Fish, Big Problem

Why minimum-size limits may encourage the harvest of our most important fish.
Large fish are far more than just angling trophies.
Illustration by Christian Northeast

As far as fish go, bigger really is better. A lot of us like to have the opportunity to take home trophy-size fish, but recent studies have concluded that large fish play a much more important role in the size structure and long-term sustainability of our fisheries. While studies have been directed at the establishment of marine reserves as the way to protect larger individuals in a variety of species, these papers also prove that current minimum-size management regimes may have unwanted consequences. They also both suggest alternative management regimes.

Ten years ago, Phil Goodyear, PhD, presented a paper at a National Marine Fisheries Service symposium on recreational fishing concerning the impact of minimum-size regulations on striped bass. He theorized that mandating a minimum size of 28 inches for striped bass, which happens to be the size where they really hit their maximum growth rate, selected the faster-growing and potentially bigger fish. Over time this minimum size would encourage the harvest of those fish that were hitting their stride and growing faster, leaving the slower-growing, smaller fish in the stock. It would also place all the fishing mortality on the spawning-stock biomass. While this paper received some press, many still felt the slowing of the growth of striped bass was due solely to lack of forage base, and was not management induced. It appears that Goodyear was ahead of the curve.

Large Role for Large Fish
At this year's annual meeting of the Marine Fish Conservation Network, the issues of size and age once again were topics of discussion with two presentations. Both papers - one by David O. Conover, PhD, and the other by Steve Berkley, PhD - came to the same conclusion that older and larger, and predominantly female, fish played an important role in the sustainability of any species. These big females are important because they produce many more eggs per pound of body mass than smaller fish. Their eggs are more viable and their larva have a faster growth rate than those of young females.

Berkley's presentation showed that a 12-inch West Coast rockfish would produce approximately 150,000 eggs, but a 28-inch fish would produce 1.7 million eggs, a dramatic difference. Older rockfish also tend to have an earlier spawning period that meshes better with the abundance of food. Unlike humans who live to a certain age then begin to die off, fish have constant low-level mortality throughout their potential lifespan and they reproduce throughout that lifespan. Berkley concluded that either marine reserves or a slot limit were management alternatives that would reduce the mortality of large female fish.

Conover presented the findings of an ongoing experiment that has spanned many years. He split a school of tank-spawned Atlantic silversides into three tanks. One school had all the larger specimens removed for five generations. Another had all the smaller specimens removed. The third was randomly harvested. After five generations, the fish that had the smaller size harvested were compared to the fish that had the bigger specimens harvested. The large fish had 54 percent higher growth efficiency. They had twofold higher fecundity or egg production. Their eggs were 18 percent bigger, and the survival rate for the larva was three times higher and the growth rate 20 percent higher.

For those species that have been managed with high minimum-size limits, the next part of the presentation causes more concern. After five generations in the tank, Conover wanted to see if and how quickly these populations would recover from the harvest selectivity. While this part of the experiment is not complete, there appears to be a trend indicating that fish recover to the norm much more slowly than they went the other way. Conover, like Berkley, suggests two possible management alternatives to help solve the problem: Institute marine reserves or slot limits.

Selection Solutions
What does this mean for fisheries management of many species that we like to catch? First, it implies that minimum-size catch restrictions may have altered the genetic make-up of these fish and affected their ability to establish a normal size distribution even if all catching ceased. It says that managers should do everything possible to protect those remaining large specimens. These fish are the key to the fastest way to recover stocks to normal growth and size patterns. Many in the marine conservation community advocate marine reserves as a way to protect large specimens in various species, but the science points to slot limits (minimum and maximum sizes) as being equally effective and far less onerous to non-commercial users.

There is one major fly in the ointment and that has to do with large-scale commercial fisheries that employ either otter trawls or gillnets to catch fish. It is hard for these methods to select for smaller fish. But it can be done easily with hook-and-line commercial fisheries.

Does this mean that all fisheries should have slot limits? Perhaps not, but it seems that all near-shore, high-abundance fisheries that are the concern of recreational and personal-use fishermen should move toward slot-limit management and hook-only harvest. The only alternative will be no-take marine reserves and those will present a whole different set of problems.