RECRUITMENT, in the context of fisheries, is the process of emergence of the next generation of fish. Obviously, this starts with spawning of the previous generations of adult fish, including egg fertilisation and hatching, through the larval and juvenile stages to the point in time when the resulting fish grow large enough to reach the minimum size, after which they can be taken by the fishery. In other words, recruitment is about where fish come from.
Clearly, without fish there is no fishing, so recruitment processes should be extremely important to anglers. Yet it seems the average angler knows little if anything about what factors influence recruitment for the species they catch. This was bought home to me recently when I fielded a question from a prominent recreational fishing representative. This person asked how many juveniles would be produced by a female flathead above 75 cm long, in the context of a discussion of the effect of a potential maximum size of 75 cm for a “slot” limit in NSW. When I replied that the answer to that question in any given year could range from zero to multiple millions of fish, I was not trying to be vague, I was simply trying to communicate a realistic assessment of the reality of fisheries recruitment.
Because it’s not a simple process. Even before adult fish get together to spawn, the abundance of food items they encounter in the months and weeks leading up to spawning can effect the quality of their eggs and sperm, which directly influences both the number of eggs that will be produced and fertilisation success. Broodstock that have been well fed tend to produce more and larger eggs of higher quality, which are more likely to be successfully fertilised. Even then, pre- spawning migrations and aggregations can be interrupted by extreme weather events and heavy fishing pressure. Even if the adult fish are not caught and removed from the fishery outright, rough handling during catch and release of a large female flathead in the hours, days or weeks leading up to spawning may cause her to resorb eggs and not participate in spawning at all (though one would hope that intelligent anglers can negate such problems by employing best practice such as keeping the fish in the water at all times, or otherwise dehooking while minimising netting damage and air exposure). So even if broodfish survive catch and release, the stress can reduce the effective broodstock population to a point where spawning aggregations become disrupted, leading to sporadic egg supply.
But it’s not all about egg numbers anyway, because after spawning and fertilisation the survival of the planktonic larval and early juvenile stages is almost entirely dependent on ambient environmental conditions. After hatching larval fish can survive on their egg yolks for only a day or two before they need to feed on zooplankton. But if zooplankton of the right size, and type are not available at high enough densities for larval fish to successfully encounter them in the correct nursery habitat, the larvae all die out within a day or three. Of course, all during this time the larvae themselves are also being eaten by other fish and crustaceans that are also in the plankton. Aquatic food chains are based on massive predation rates, so even in the best of conditions with everything firing on all cylinders, to expect survival of more than one per cent of fertilised eggs through to the juvenile stage would be extremely optimistic.
So how can we ensure that there is enough zooplankton out there in the right places to feed our flathead larvae? Well, to do that there needs to be just the right amount of nutrients in the water in the nursery areas to generate the right phytoplankton blooms for those zooplankton to eat. To get that phytoplankton bloom we might need the right river flow event a month or three earlier, or the right coastal upwelling event, which in our part of the world is usually driven by offshore wind events. In other words, the number of flathead larvae that survive to become early juveniles (which all going well might be able to recruit to the fishery in two or three years time) might not even depend on the number of spawning flathead at all. It might depend entirely on the weather.
This sobering point was bought home recently by an excellent study by Hayden Schilling and colleagues from the Sydney Institute of Marine Science. Their paper, entitled “Coastal winds and larval fish abundance indicate a recruitment mechanism for southeast Australian estuarine fisheries”, examined the relationship between coastal wind events and recruitment of various species of estuarine fish, including not only dusky flathead, but also yellowfin bream, yellowfin whiting and sea mullet. The paper was published in Fisheries Oceanography, a scientific journal dedicated to reporting results of these sorts of fascinating studies from around the world.
Schilling et al. found that coastal winds can influence the abundance of these species at levels which resulted in detectable effects on commercial fisheries catch. They found evidence for higher larval numbers recruiting into estuaries following a predominance of onshore SE or easterly winds (= downwelling winds), due to wind-driven transport of the larvae inshore. However, the effects of this retention of larvae closer to shore were most prominent if they followed previous offshore (westerly) winds patterns a few weeks earlier, which most likely primed coastal ecosystems with nutrient rich water from upwellings, thereby creating favourable plankton blooms near the juvenile nursery areas. Their data analysis also found that in southeast Australia onshore winds have decreased since 1850, which may have reduced larval recruitment to estuaries in recent times. They concluded by stating that future research on the recruitment of estuarine fish in this part of the world should incorporate analysis of coastal winds.
This sort of research is fascinating as it can explain quite a lot of what we see on the water. For example, in northern Moreton Bay now that the Brisbane River is dammed for Brisbane’s water supply, the majority of water in the bay flows with the run in tide from the northern entrance of the bay between Bribie and Moreton Island. This water tends to circulate in a clockwise fashion and bounces off Redcliffe before exiting the bay via Skirmish Point on the run out tide. Under these hydrodynamic conditions the prevailing SE winds tend to push fish larvae towards the north west, which as it turns out encompasses some well-known fish nursery areas such as Deception Bay. Historically, the shallow flats of Deception Bay had huge areas of seagrass which provided high quality nursery habitat for larvae and early juveniles of key estuarine species such as bream, whiting and flathead. However since the mid 1990’s the effectiveness of Deception Bay as a nursery habitat has declined markedly after Lyngbya algal blooms began to kill off huge areas of seagrass. Research demonstrated that these Lyngbya blooms were fuelled by nutrient loading and increased bio-available iron caused by disturbance of acid sulphate soils from nearby urban developments. So when it comes to nutrients, it’s certainly possible to have too much of a good thing. This example of the decline of the nursery function of Deception Bay (the loss of seagrass has greatly reduced the area available for larval fish to settle from the plankton) is a warning of how finely balanced the factors influencing fish recruitment are.
Of course, we can’t control the weather, so what else can anglers do? We can work on habitat to ensure that the critical nursery areas like seagrass beds and inshore reefs remain functional for the larvae to settle on. These same areas also provide habitat for juvenile fish to live in and feed while providing protection from larger predators as they grow. Of course we can also try to maximise the number of fertilised eggs entering the system. This is so that during those good years when all the planets align and the right plankton blooms occur in the right places at the right time, we have a chance to get hundreds of millions of larvae into these nursery areas in the hope that in two-to-three years time, millions of juveniles will recruit into our fisheries. So great fishing is all about planning ahead to facilitate successful recruitment, so we can get the ecosystem firing on more cylinders so we can make the most of the good years. Which is also why a slot limit for flathead with a 75 cm maximum size seems a great idea.
The paper by the Sydney Institute of Marine Science researchers is free to download and read at: onlinelibrary.wiley.com/doi/epdf/10.1111/fog.12561
