A STATEMENT along the lines of “why do some fish grow faster than others” seems, at first
glance, to be obvious and self evident, given the many thousands of different species of fish. However, when referring to the growth of individuals within a single given species, this is not
a silly question to ask after all.
The fact that some individual fish grow much faster than others of the same species (also
known as their conspecifics) within their school or in a given water body has been known for
many decades. Indeed, this fact has become increasingly evident as aquaculture and the
science of aging fish have both developed.
The ability of an individual fish to be able to grow faster than others in its cohort or year class certainly has important ramifications for natural selection. The very survival of fish during their early life stages in the plankton depends entirely on a combination of its ability to avoid predation while finding enough food to grow larger (and thus become less vulnerable to predation). This natural aquatic law of “eat, or be eaten” is demonstrated widely by the many fish species which cannibalise their conspecific buddies in order to outgrow them, as anyone who has reared predatory species like barramundi would know. Many a baby barra has choked and died trying to eat one of itssiblings, but if the greedier fish manages to swallow its first piscivorous meal, the resulting growth spurt provides a huge boost to its chances of eating the sibling next door.
Under almost all natural conditions, an individual fish is much more likely to survive and reproduce if it grows big and does so quickly, yet wild populations of a given fish species will harbour many different genetic variants. Some of these genetic variants will have different physical, or physiological characteristics (such as a larger mouth, or a faster rate of digestion). It is these genetically induced variations which provide the population as a whole with resilience to changing conditions, which is important for natural selection of the population over longer timescales.
So some of the variation we see in growth between individual fish of a given species is undoubtedly of genetic origin, yet aquaculture has shown that even when genetic diversity is
minimised (e.g. by breeding from selected broodstock) and the resulting fish population is reared in the same environment with the same food supply, some individual fish will still grow faster than others. In these cases, the differences are often due to behavioural variation between “bold” fish which are more active and aggressive, and “shy” fish which are more submissive, introverted and, therefore, less dominant. Studies have found that there can be multiple reasons for the development of “bold” and “shy” fish, including parental, environmental, morphologic and metabolic variables, as well as social interactions.
A recent review of the various factors which interact to influence the growth rate of individual fish was presented in a paper by Goodrich and Clark in the journal Fish and Fisheries. In it the authors explore how some of these influences on fish growth (beyond the obvious genetic ones) occur even before spawning, with parental condition, nutrition and stress influencing factors such as the size of the fish eggs prior to fertilisation. Egg size in turn directly influences the survival rate of eggs and larvae, as those with larger yolks have greater reserves which allows the fish larvae a longer window of time to find their first feed.
The type of first feed as well as the water quality to which the fish larvae are exposed also play an important role by influencing the bacterial flora of the intestine (the microbiome) of individual fish, which in turn can influence gut morphology, digestion and metabolic rates.
Those fish with larger guts and/or more efficient gut metabolism in turn tend to be “hungrier”, which can also influence their behaviour as they need to feed more often, which in turn may make them more aggressive feeders. It is through one or a combination of these factors which usually explains why some fish grow faster than others.
On the other hand, in the modern world there are new forces at play in the form of human induced selective pressure which have changed some of the rules of the game. For example, heavy fishing pressure of a fish species which has a minimum size limit set below the size at which it matures, can result in increased mortality of faster growing fish, leading to favouring of fish which may grow slower or mature at a smaller size. In the absence of a minimum size limit, heavy fishing pressure of fish of all sizes usually favours those individual fish which grow fastest and mature early. In both cases, this so called “ fisheries induced evolution ” imposed over many generations of fish can result in an overall reduction in the size of mature adult fish. This is a problem, because the fecundity of fish (the number of eggs produced by females) is directly (and usually exponentially) linked to the size of the female fish. So even though the number of fish in a given population may remain unchanged, fisheries induced evolution can still greatly reduce the reproductive potential of that fish population.
The realisation of fisheries induced evolution has been one of the justifications used by some
interest groups for the establishment of marine protected areas which ban fishing. But of course there are other ways of preventing the negative effects of fisheries induced evolution. This is particularly so in recreational fisheries where enforcement of regulations such as slot limits, maximum sizes, and catch and release can be used to minimise the detrimental effects of fisheries induced evolution. While in commercial fisheries, elimination of overfishing has been found to be a critical factor in reducing fisheries induced evolution.
For more information on the many factors that influence the growth of individual fish, check
out the review by Goodrich and Clark at: https://doi.org/10.1111/faf.12770 , while some
information on fisheries induced evolution can be found in the papers by Hollins et al.
( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5978952/ ) and Hutchings and Kuparinen
( https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12424 )
