A team of scientists has now confirmed that size really does matter – at least, it does when talking about fish moms. Scientists have long understood that larger females produce more eggs; however the question of just how many more eggs have been up for debate. While it may seem trivial, understanding how the size of a female corresponds to the number of offspring she will have is a huge issue when considering ways to regulate fisheries and conserve species. When thinking about a fish population, biologists have to consider how removing fishes will impact the entire fish population. For instance, catching a reproductively active female out of a population means that you are not just removing one fish from the population, but rather, you are removing her and all of the babies she could have had, which would have added to the fish population in the area. We know that there is a demand for larger fish – after all, everyone wants to have a big fish story tale. But those bigger fishes are also better moms. The question is just how much better is a bigger momma?
For many species, fisheries models typically assume that there is a linear (isometric) relationship between fish size and reproductive output. That means that a one pound fish will have half the amount of babies as a two-pound fish. So, we only need two one-pound fishes in the population to make up for a two-pound fish being caught and removed. The math seems simple enough (1+1 = 2), but the concept might not be fool-proof.
To try and figure out if fish reproduction scaled with body size, scientists had to dive into the literature. The team of researchers parsed through different reports of fish reproductive output as it scaled with body size and compiled a bank of data on a total of 342 marine fish species. The researchers used these data to examine the link between female body weight and the amount of energy put towards reproducing (reproductive output). To calculate reproductive output, the researchers had to consider not only how many eggs a female had, but also how large those eggs were and how much energy was contained in those eggs. Even if a female had double the number of eggs, if those eggs were half the size or contained half the amount of energy, her offspring might not survive as well as a mother with larger eggs and more energy stores. By multiplying the total number of eggs by the size of the eggs by the total energy in each egg, the researchers were able to consider more accurately how a mother would reproductively contribute to the population (reproductive output = number of eggs x egg size x egg energy).
Upon analyzing the data, the researchers found that the majority of fishes did not display an isometric relationship between size and reproductive output. In fact, in 79% of the species examined, bigger moms had a disproportionately larger reproductive output than smaller moms (hypergeometric scaling). So, it would take more than just two one-pound fishes to produce the same amount of babies as one two-pound individual (1 + 1 ¹ 2).
An example with Atlantic Cod:
The Atlantic cod (Gadus morhua) is a critically important fish in the Northwestern Atlantic. The species sustained heavy population depletion due to over-fishing, and the fishery for the species has been carefully monitored since the population collapsed in the 1980s. If assuming that fish size scaled isometrically with reproductive output, the reproductive output of a 30-kilogram fish could be matched by just 15 2-kilogram fish. However, according to the analysis here, scientists found that it took almost double the number of 2-kilogram fish to match one 30-kilogram individual (it would take more than 30 2-kilogram fish).
Knowing that larger females are far more important to the reproductive success of the population can have huge implications for how cod is managed and may inform fishing regulations.
What can we do with this information?
In addition to informing fisheries management strategies, the authors also point out how important this information could be for conserving key marine habitats and setting up marine protected areas (MPAs). Because MPAs help to protect fish by allowing many species to reach larger sizes, the protected areas could have a much larger impact on conserving populations than previously acknowledged by acting as a refuge for bigger mothers that produce more offspring. As mentioned in the case of the cod, this information could have important implications for how we manage fisheries. For instance, regulations may include size restrictions aimed at protecting larger females.
Knowing how size impacts reproductive output will also help scientists predict the fate of future fish populations. We know that overfishing has been reducing the size of many fishes. As we preferentially remove the larger fishes from a population, it leaves smaller and smaller fishes behind to reproduce. Because it is disadvantageous for a fish to be large, over time we have seen some species of fishes reduce their size overall, a phenomenon referred to as fisheries-induced evolution. Knowing that smaller females contribute fewer offspring to the population means that over-fishing could be contributing to a cascading effect where fisheries are not only removing larger, more productive females from the populations but are also stunting the reproductive productivity of future generations by selecting for smaller individuals in the next generation.
On top of facing population depletion from over-fishing, fish are also battling warming ocean temperatures due to global climate change. Warmer temperatures have been predicted to decrease the size of some fish species. Along with this decreased size, the researchers predict a larger decrease in reproductive output based on their analyses here.
While it may seem like the deck is stacked against fish populations right now, a more accurate understanding of how individuals contribute to the future generation could help turn everything around. This information could help scientists more carefully manage fish species and predict how different actions will impact future fish populations.