Big Female Fish
There’s a common misconception within the angling community that fish reproduction follows a similar trajectory as humans or other mammals. You know the story…fish become sexually mature at a certain age, they reproduce like gangbusters, and then at some age, female fish produce less eggs, and those eggs really aren’t that good. Right? After all we mammals have biological clocks, why shouldn’t fish.
There’s a term that scientists use called “Big old fat fecund female fish” or BOFFFF. BOFFFF is a hypothesis that has been tested on a wide variety of fish species, and the results have shown that for many species, older female fish reproduce more and produce better eggs and better offspring.
Some of the reasons just make sense. Larger fish have more space to store eggs. And, because they are growing less, they can devote more food energy to reproduction. Did you know that just one 30-inch Red snapper will produce as many eggs as 100 13-inch Red snapper?
But what about the quality of those eggs you ask? Well, it turns out bigger fish produce bigger eggs. And, bigger eggs produce bigger larvae. Bigger larvae are more resistant to starvation and predation, because they tend to have a bigger yolk, which equates to more food for the developing offspring.
But, there’s more good news for offspring of big fish. During the final stage of egg development in most marine fishes, there is a massive uptake of water. This increase in water is a survival adaptation that makes the eggs of pelagic spawning fish neutrally buoyant. Being neutrally buoyant helps eggs released into the water column during spawning reach suitable habitat for optimal growth and survival. Guess what? Yes, bigger fish have been shown to produce more buoyant eggs.
Not only do bigger fish produce more eggs, bigger eggs, more buoyant eggs, and healthier larvae, they also spawn more times over the course of a spawning season. In the case of redfish, a recent study showed that first year spawners didn’t even join the spawning aggregation until the end of the spawning season. By this time, much larger redfish had spawned several times. More spawns mean the bigger fish are spreading their eggs over many more locations and habitats than smaller spawning fish.
BOFFFF has major implications in fisheries management. In fisheries where BOFFFFs are not protected, we may be selecting for smaller spawning fish. This happened in the Atlantic cod fishery and it certainly could happen elsewhere. The loss of big female fish impacts the size and age structure of fish populations.
OK, but is there a point in which old fish are just too old to reproduce or at least too old to do it well? Well, this has been seen in some small aquarium fish like guppies and zebrafish but it is believed to be quite rare in any fished stock due to how quickly age truncation occurs. In red snapper for example, the reproductive lifespan is estimated to be 49 years, however very few fish over the age of 21 have been observed in the stock throughout its range.
Another complicating factor is that big doesn’t always mean old. In fact, aged snook greater than 38 inches, have been found to be from 6 to 19 years old. That’s a crazy spread! So, if fisheries managers were to set the slot too high, they would risk removing a lot of big young fish with many years of reproduction left.
Another issue that complicates things for snook is that big doesn’t always mean female. Snook change sex from male to female in what’s called protandrous hermaphroditism. However, while most males change sex, not all do, and they certainly don’t do so at a specific length or age. So, in the same group of 38+ inch snook as mentioned above, approximately 15% were males.
This all seems quite complicated, but it doesn’t have to be. Just remember, smaller spawning fish produce fewer offspring that are less fit with a slimmer chance of survival. Over time this means fewer adults spawning fewer fish. This is not a good cycle, and it’s one that would be very hard to reverse. So, catch phrase of the day: BOFFFF’s are the future of our fishery.
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