These same big, old fish are also routinely sought by fisherman, scientists say, and the age decline in fish populations helps to explain the collapsing fisheries off the Pacific Northwest coast.
Other research also indicated that many fish populations up and down the coast are essentially distinct, and fish from one area don't intermingle as much as had been assumed with fish from elsewhere.
These combined problems might be most effectively addressed with a diverse network of marine protected areas, said scientists at Oregon State University.
The findings were recently summarized in an article in Fisheries, a professional journal.
"We've known for a long time that bigger fish produce more eggs, that we might need to have numerous smaller females to produce as many eggs as one larger fish," said Mark Hixon, an OSU professor of zoology. "Modern fish management is based on this assumption. But we've also assumed that one fish egg is just as good as another, and the newest studies are showing that's just not the case."
New studies by OSU researcher Steven Berkeley, who is now at the University of California – Santa Cruz, have shown that eggs from very old fish have much larger oil globules in their yolk, giving the larvae that develop from these eggs a chance to grow faster and survive starvation longer. Older fish also spawn earlier, which sometimes better coordinates larval birth with peak food availability.
A marine ecosystem routinely has more than 99 percent mortality of fish larvae due to predation, starvation and fluctuating ocean conditions. So anything that helps young larvae pass through their most vulnerable lifestyle stages can significantly increase their chance of survival, scientists say.
"In some cases, it appears that almost all of the surviving larvae have come from large, old, fat fish," Hixon said. "For effective replenishment of our groundfish stocks, these older fish may be essential. But with the fish management systems we now have in place, fish of this age range may represent far less than 5 percent of the total population of a species."
Black rockfish, for instance, can begin to reproduce at about five years of age, but need to be about 12 years old with heavier amounts of fat before they can produce the type of "oily" eggs that may have increased survival chances. Larvae from the oldest fish can survive starvation 2.5 times as long as those of the youngest, and grow three times as fast on the same diet. Similar findings have been made with other species.
For a fish, surviving to adulthood is not easy. But after that, the natural dangers decline.
In a natural system, old fish actually have an extremely low rate of mortality compared to their younger counterparts, and a disproportionately higher level of body lipids – they get fat. Young fish are comparatively lean. And fishing pressure works exactly to the opposite of most natural mortality agents.
During a period of intense fishing off the Oregon coast from 1996 to 1999, the average age of mature female black rockfish declined from 9.5 to 6.5 years – in a fish species that has a maximum lifespan of about 50 years. Soon after, many fisheries were in serious decline.
Genetic studies by OSU researcher Michael Banks and his graduate students have also shown that there are many unique fishery stocks up and down the coast that do not interbreed, meaning that a collapse of a fishery in one location may not be easily corrected by migrating fish from other areas.
There are few easy remedies to this problem, Hixon said.
Complete closure of fishing or extremely low fishing quotas would be one approach, but this has economic repercussions and is often not acceptable to the public, he said. Requiring "catch and release" for large, older fish is not practical for most groundfish species, because their swim bladder often ruptures or other trauma occurs when they are brought to the surface, and they die anyway. "Given the demand to have a sustainable fishery and the biological constraints of most marine groundfish species, the option that seems to make the most sense is marine reserves," Hixon said.
A well-devised network of marine reserves, Berkeley, Hixon and colleagues said in their report, would allow a much higher population of large, old fish within those reserves, a higher level of larvae survival, and provide the ability of reserves to help replenish other marine areas nearby. If properly located, reserves might also address the distinct nature of groundfish populations in different geographic locations, they said.
Exactly that type of "seeding" effect has now been documented with some populations in the Georges Bank off the East Coast, Hixon said, which had huge closures in the mid-1990s due to collapsing fisheries.
"As we come to better understand the biology of these fish populations and what may have led to their dramatic decline, more and more people are realizing the role that selected reserves could play in addressing some of these problems," Hixon said. "They offer some benefits that frankly cannot be found with any other management option."
By David Stauth, 541-737-0787
SOURCE: Mark Hixon, 541-737-5364