Does size matter? (I’m asking for a friend.)
Well, according to the scientists, it matters a lot, particularly for fish.
A new literature review commissioned by Good Fishing by University of Auckland’s Dr Arie Spyksma offers a deep dive into the pivotal role that large snapper play in maintaining the balance of kelp forest ecosystems, and the far-reaching impacts of fishing.
You really should read the whole thing, but here’s the gist...
Kelp forests are among the most productive and dynamic ecosystems on the planet. They provide habitat, food and nursery environments for a host of marine species. However selective fishing over a century has removed large fish from the ecosystem, allowing sea urchin numbers to explode and mow down the kelp. This ‘trophic cascade’ results in urchin barrens—areas devoid of kelp and teeming with urchins, a disastrous outcome for reefs and the carrying capacity of the marine environment. You probably know what these look like:
Nordic Reef, Leigh Coast.
Big fish, less kina
Spyksma's review highlights the importance of body size in snapper, which determines the size and quantity of urchins a single fish can consume. Big snapper can exert significant top-down control that helps maintain the health and coverage of kelp forests. This is especially true along much of the north-eastern coast of New Zealand where the other kina predator, crayfish (red lobster), has been fished to functional extinction.
“In the absence of lobster, snapper are potentially the only remaining control of any significance on the continued expansion of urchin barrens, at least in northern parts of the country,” writes Spyskma.
Analysis of shell middens has showed that the average size of snapper caught and kept by pre-European Māori was half a metre long. So there’s no doubt the ecosystem was in better shape back then. This photo was not of course pre-European—it was shot in the Bay of Islands, about 1910. Cheers to National Library.
In areas where crayfish numbers are low, almost all predation of urchins less than 60mm in size was attributable to large snapper.
However, overfishing has also led to a marked decline in both the abundance of snapper and their size distribution, now at around 80% of their pre-human abundance. This has had dire consequences for healthy reefs, he says.
“The loss of larger individuals from the population due to fishing is likely to have had a significant impact on the effectiveness of snapper as a key predator, lowering their overall functional importance within kelp forest ecosystems,” he writes.
Big fish, more fish
Beyond their ecological role, large snapper are also vital for the reproductive success of their populations. Larger female snapper produce exponentially more eggs compared to their smaller counterparts.
A 25 cm snapper will produce an estimated 80,000 eggs over the spawning season, while a 50 cm snapper is good for 6,100,000 eggs, a 7000% increase in egg production with a doubling of body length. These eggs are not only more numerous but also of higher quality, with better chances of survival and development. This phenomenon, known as hyperallometric scaling, means that the reproductive potential of a population is significantly enhanced by the presence of larger females.
By this logic, you could pull 70-odd legal snapper out of the water and the fishery may still be better off than if you’d decided to keep that breeder.
Selective removal of large snapper undermines the species' reproductive output, leading to a decline in numbers over time, and coupled with their role controlling urchins, can lead to a rapid collapse in reef function.
Catch and release fishing is becoming increasingly popular, particularly among avid fishos. All of these giant fish were caught in shallow water, properly handled, safely released and swam away to live and breed another day. Read more about catch and release survival rates.
Most strangely, the long-term act of removing large fish from an ecosystem also results in ‘fisheries induced evolution’.
“Intensive fishing pressure has been shown to favour the evolution of faster life histories whereby fish mature younger, investing more in early reproduction, and then experience post-maturity declines in growth rate,” writes Spyksma.
The outcome is a fisheries-induced shift towards smaller, slower growing snapper within the New Zealand stocks that have been most heavily fished.
How do we get the big fish back?
Spyksma’s literature review concludes with several recommendations for improving the management and conservation of snapper populations.
Implement Size and Catch Limits: Set a maximum size as well as a minimum size (called a slot limit) to allow large snapper to fulfill their ecological and reproductive roles. We will also need to allow commercial fishers to sort landed catch for max size if it will survive.
Establish marine protected areas: Utilise MPAs to conserve critical habitats and allow fish populations to recover. The Hauraki Gulf Marine Protection Bill is imperfect but it’s a good start.
Catch and release the breeders: In most cases, we discovered that survival rates are high. So Instagram the big ones, release them, and keep the pannies. If everyone did this the fishing would dramatically improve.
Read The Value of Big Fish, Dr Arie Spyksma’s full literature review.
