Broadcast spawning might be well known, but fish sex is much wilder, more diverse and often much more fascinating than most people know. I’m talking sex changes, male pregnancy, complicated dating protocols, and – most human-like of all – a tremendous investment in individual offspring. A catalogue of some of my favourites will follow. Before we go there, it is worth pondering the fact that fish are in fact wildlife – like quokkas or Tasmanian devils. But, unlike land-based wildlife, they are targeted for food and recreation to the extent that in Australian populations about one third of fished species is either overexploited or its status is unknown. So sex is important because it is through reproduction that animal populations replenish themselves. If we humans overly disrupt reproductive output through fishing, we undermine the ability of fish to maintain their populations. So we need to understand how fish have sex to make sure we don’t mess it up.
The tale of dhufish courtship reads much like a sudsy Mills and Boon novel. Found uniquely in Western Australian waters, this favourite target of commercial and recreational fishers is slow-growing, long-lived and has one particularly endearing quality: the big males have small testes relative to their size. In the world of fish, small balls typically indicate that you have impressed your females with good overall genetic material and chivalrous behaviour (chocolates and roses).
In other words, you don’t have to get out there and compete with a lot of other males. At the height of breeding season, a big mature female will be carrying a kilogram of mature eggs, compared to a mature male’s 100 grams of testes – a small fraction of her reproductive effort. We deduce from this that the gals invest this energy only when attractive (big, mature) males are present (sound familiar, ladies?). With commercial and recreational fishing reducing the abundance of large male dhufish, preliminary evidence suggests that some females are not ‘readying’, or hydrating, their eggs during the spawning season, keeping them instead in a kind of suspended animation as they wait in vain for Mr Just Right. We may well be creating a lonely-hearts club in the waters off WA, with female dhufish reproducing irregularly for lack of big males.
As a fish, you can choose to have lots of tiny offspring with high mortalities or a very few big ones that are likely to make it to adulthood. While many fish have evolved to produce millions of eggs annually, white sharks are the most extreme contrast to this. They make a huge investment in energy because they only produce small numbers of really large offspring. While a clownfish produces 200 to 1000 larvae, of which 99.99 per cent will die before they reach adulthood, white sharks bank on most of their offspring surviving. Once white shark females reach approximately 4.5m in size, they produce between two and ten pups every two to three years. These pups are typically between 1.1m and 1.7 m long, which means the smallest litter is about eight per cent of mum’s body weight. Contrast this to humans where an average baby is equivalent to about four per cent of a typical Australian mum’s weight. White sharks invest heavily in individual pups to maximise their survival. The problem is that they don’t count human factors such as hunting in the arrangement; it is their extremely slow reproductive rate that makes their populations so sensitive to human intervention. Considered by the International Union for the Conservation of Nature (the gold standard for assessing conservation status) as globally ‘Vulnerable’, they are totally protected in Australia.
And now to clownfish, the protagonists in the animated movie Finding Nemo.
Gender dysphoria can be tough for humans, but spare a thought for Nemo, whose dad was in fact his mum... or his wife. These fish begin adult life as sexually indeterminant juveniles, living in a sea anemone nest with a dominant large female and her subordinate, smaller male mate. Mum lays her eggs on a hard surface underneath the anemone’s protective canopy, dad bathes them in his manly essence, and then spends two weeks caring for the now fertilised 200 to 1000 eggs until they hatch and leave home to float as part of the plankton, eventually settling in their own nests as juveniles. The kicker is that should mum be captured for the aquarium trade, dad changes sex and becomes a female while the most dominant juvenile from the litter becomes a male, ‘Dad’s’ new partner.
Australia’s iconic gropers do the complete opposite to clownfish. Australia’s largest cold-water fish and unique to Australia, Western blue gropers are
wrasse found in small groups comprising a dominant big blue male, plus several smaller green females and juveniles. When the harem’s male is lost to fishing or old age, the dominant female changes sex and colour. As this change typically occurs at around 30 to 35 years of age and 0.8 m in length, it is not clear how fishing big blue males, through, for instance, spearfishing, affects these populations. However, concerns about sperm limitation exist for other fished wrasses and we need to ask: can young, undersized females step up on the loss of the male, or does the harem have to sit tight until a harem female reaches appropriate size and age, or another age-appropriate female arrives? In either case, reproductive output is likely slowed.
Seahorses do things very differently to gropers. They demonstrate that the ability to withstand pregnancy and birth isn’t limited to females. Seahorses form monogamous pairs with very involved courtships. The male finds his partner each day by flashing his colours. They ‘stroll’ together through the seagrass, their tails entwined. For his troubles, she deposits her eggs in his breeding pouch where they are internally fertilised. The embryos attach to the inside of the pouch, where they receive nutrients and develop into fully formed replicas of an adult seahorse, albeit only millimetres high. About two weeks later, the male labours earnestly to eject several hundred young from his pouch. The good news is the baby seahorses immediately leave home. The bad news is that Mum is typically ready to go again. The worst news is that the capture of a partner for the aquarium or for the Chinese medicinal trade leaves the remaining animal high and dry, and it is not clear how quickly he or she can establish a new reproductive partnership to contribute to population numbers, given the animal’s strong monogamy and limited ability to swim and search out new partners.
So the many and weird ways that fish reproduce can make them exceptionally vulnerable to fishing. If we don’t understand their reproductive behaviour, we can’t understand the way in which fishing will affect their populations. To this end, marine parks – closed to all fishing – allow us to learn about these animals in an unexploited environment so we can better manage them in areas where fishing occurs, and also provide a sanctuary.
Professor Jessica Meeuwig is Director of the Centre for Marine Futures (Oceans Institute), at the University of Western Australia.