Coastal fisheries ecology

Fisheries have removed the very large, mature, fecund fish and fewer parental fish usually means fewer offspring. Sometimes through oceanographic luck there may be a larger than average survival of eggs and larvae. One consistent predictor of larval supply could be the growth of larval fish. If larvae grow quickly through the highly vulnerable, high-mortality larval stage they are less likely to die. This “stage duration hypothesis” is the focus of our research.

The motto for larval fish could be “Grow or die”!

How do we measure growth? All bony fish or teleosts have 3 pairs of ear-bones or “otoliths”, as part of their balance organs. Others describe them as accelerometers for pitch, roll and yaw in a viscous, three dimensional world. These bones grow in daily (and annual) increments by an endogenous, circadian rhythm. The otolith size is usually in proportion to fish length, so we have a daily measure of actual growth.

The otoliths of a larval fish are circled, and one is shown in the lower left (only 0.2 mm in diameter). But juvenile fish are hard to sample in the open ocean. We acquire the “teenage” fish from the fishery, at least a year before they are properly harvested, to provide management with information on larval growth – which is our abundance index.

Larval and juvenile fish are expensive to sample, but juvenile pre-harvest fish are routinely caught by the fishery. Their growth could be a correlate of survival and therefore abundance as shown below:

Further reading:

Uehara, S, A Syahailatua and IM Suthers. 2005 The δ15N and δ13C signatures of faster growing larval pilchards, Sardinops sagax: contrasting effects of upwelling by the East Australian Current. Marine & Freshwater Research 56: 549-560.

Zooplankton Biomass Size spectrum

Size of an animal is a useful correlate of longevity, metabolic rate, consumption. The size frequency distribution of animals in the ocean, from bacteria to whales is known as the Biomass Size Spectrum. It suggests that the total biomass of bacteria used to be twice that of whales! More practically we use this approach for the copepod to krill size range – with other gear we can include bacteria, phytoplankton and protozoa.

There is no better place in the world to study the links between nutrients, phytoplankton, zooplankton, larval fish and fisheries than in the East Australian Current, which is comparatively so close to land. Australia’s fisheries have the lowest yields in the world by far and yet fishing is an essential part of our society (20% of Australians fish at least once per year). The cause of the Australia’s low yields is presumably related to our clear nutrient starved ocean waters and narrow continental shelf, especially off NSW.

The figure illustrates the dual affects on the slope and intercept of the “spectrum”. Bio-mechanical models of this, driven by nutrients and linked to oceanographic models such as BlueLink will provide a weather-map-style prediction of plankton dynamics. We measure nutrients from a bottle sampler (right) and plankton size on a towed instrument (left).

Further reading:

Baird, ME and IM Suthers 2007 A size-resolved pelagic ecosystem model. Ecol. Model. 203: 185-203

Suthers IM, Taggart CT, Rissik D, Baird ME. 2006. Day and night ichthyoplankton assemblages and the zooplankton biomass size spectrum in a deep ocean island wake. Marine Ecology Progress Series 322: 225-238.

Moore, SK and IM Suthers. 2006. Evaluation and correction of subresolved particles by the optical plankton counter in three Australian estuaries with pristine to highly modified catchments. Journal of Geophysical Research 111, C05S04, doi:10.1029/2005JC002920

Perhaps we can avoid the annual gamble of larval survival and artificially propagate juveniles at a cost-effective size and re-stock into some key, estuarine locations. The 26 newly declared NSW recreational fishing havens are logistically convenient arenas to test this 100 year old concept in some novel ways. It is astonishing that prior to our research program there was no ecological method available for determining how many fish should be released.

Is an estuary food, habitat or nutrient limited? Some of our most exciting discoveries in the Georges River show that we need to release mulloway at particular times, at particular places and at densities estimated from habitat and food availability – or “targeted re-stocking”. A similar approach is being developed for eastern king prawn and Australian bass.

Can hatchery-reared species be trained to recognise wild food items upon release? Survival of released fingerlings and prawns may be enhanced by sending hatchery-reared fish to “school”, where they learn to deal with the conditions in the wild.

Further reading:

Taylor MD, and IM Suthers. 2007. A predatory impact model and targeted stocking approach for optimal stocking of mulloway (Argyrosomus japonicus). Reviews in Fisheries Science, accepted 20 April 2007

Taylor MD, S Laffan, IM Suthers and DS Fielder. 2006. Key habitat and home range of mulloway (Argyrosomus japonicus) in a south-east Australian estuary: Finding the estuarine niche to optimise stocking. Marine Ecology Progress Series 328: 237-247