Hoki: a case study
Hoki (Macruronus novaezelandiae) is the most abundant commercial fish species in New Zealand waters. Since 1995, however, there has been a below-average survival of young fish that contribute to the hoki fishery. This low recruitment along with high levels of fishing has reduced the western stock of hoki to about one-fifth of its original level. A major study between 1979 and 1989 helped scientists determine the conditions that affect the growth and survival of hoki larvae.
Birth and growth
Mature hoki live in deep water and migrate to special spawning grounds around New Zealand. In winter one of the places they head to is the waters west of the South Island, where they release their eggs above the continental slope at a depth of about 200 metres. The fertilised eggs float to the surface and become part of the plankton. The eggs take three days to hatch, and it is another five days before tiny larvae develop mouths and begin to feed.
To escape predation hoki larvae gorge themselves and grow quickly: big mouths allow them to eat relatively large food items straightaway. They favour the tiny (1 millimetre) crayfish-like zooplankton called copepods, especially Calocalanus. Oceanic, warm-water forms of Calocalanus thrive just off the West Coast in some conditions.
In the Tasman Sea in autumn, surface waters mix with deeper waters, and the amount of phytoplankton increases rapidly. Copepods graze on this new organic matter and are in turn eaten by hoki larvae.
However, there are subtle, year-to-year differences – driven by the weather – in the timing and pattern of water mixing, which means that luck plays a large part in whether larvae get enough food. For example, in 1987, when mixing started early and progressed slowly to deeper water, phytoplankton and copepods were abundant. Hoki born in 1987 were strongly represented in the adult population some years later. In contrast, in 1990, when mixing started late and progressed rapidly to deeper water, it was a poor year for hoki larval survival. There was not enough phytoplankton to support copepod growth to the concentrations required by hoki larvae.
But this is not the whole story. From 1996 to 2002 the Tasman Sea warmed up to at least 800 metres deep. This coincided with a low recruitment of juvenile hoki into the adult stock. It is not known why numbers declined, but it may be connected to reduced nutrient concentrations, changes in the depth of mixing waters, or a combination of these factors.