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Big Fish Are Hefty Contributors in Replenishing Fish Stocks

Even accounting for their proportionate size, bigger female fish produce many more offspring than smaller fish, a new study in the 11 May issue of Science reveals. The results hold implications for fisheries, where large fish are often preferentially harvested, potentially creating a cycle where fish stocks perpetually dwindle.

Fisheries models assume that the reproductive output of one 2-kilogram (4.4 pound) fish is equal to that of two 1-kg (2.2 pound) fish; yet field biologists have repeatedly suggested that the number of offspring that a fish produces may increase disproportionately with body mass.

Atlantic cod (Gadus morhua) | NOAA

Diego Barneche of Monash University, an author involved in the Science report, noted that a few studies have confirmed that in some species larger female fish have greater fecundity. "But it was unclear whether this pattern was common. So we decided to dig deep into the literature to bridge this gap using as much as data as we could find."

His team analyzed the total reproductive-energy output of 342 species of marine fish. They studied how many eggs a female produces given her size; how the average volume of eggs changes with female size; and how the amount of energy (similar to a measure of calorie density) contained within an egg changes relative to egg size.

In the majority of species, the researchers found a scaling pattern, called hyperallometry, where bigger mothers produce exponentially more offspring. For example, on average a single 30-kg female produces more eggs than about 28 2-kg females that collectively weigh 56 kg. A batch of eggs from a 30-kg female harbors 37 times the total energy content of a batch from her 2-kg counterpart.

Dustin Marshall, also at Monash University and a co-author of the study, said he was surprised how widespread this pattern is. "Our results show that hyperallometric reproduction is the rule, not the exception, and that in the absence of any other information, hyperallometry should be assumed."

He noted that most classic fisheries models do not account for this disproportionate contribution that larger fish make in replenishing populations, yet these are the first individuals to disappear under even moderate fishing pressure. "So fisheries scientists, despite the best of intentions, have been using models that inadvertently recommend overharvesting," Marshall explained.

Another major concern is that climate change is expected to drive a reduction in fish size — which in turn may mean even fewer fish. For example, the estimated size reduction of the Atlantic mackerel (Scomber scombrus) that is predicted to occur in the face of a global average temperature increase of 1.5⁰Celsius (2.7⁰Fahrenheit) would result in a 50% per capita reduction in fecundity for the species.

While the results of this study raise concern over the future of fisheries, they also point to one way to mitigate the decline of large fish and fish populations. Marine Protection Areas (MPAs), which either strongly restrict fishing or ban it completely, offer a sanctuary where fish can safely grow to larger sizes.

Marshall said, "Fish grow to around double the mass inside MPAs — our results show that this doubling of average size increases fish reproductive output by as much as 74% inside MPAs, providing more recruits and replenishment for the harvested populations outside the MPA."