By Paul Franklin
Every spring New Zealanders can be found creeping out at the crack of dawn to line the lower reaches of our rivers in the hope of catching that New Zealand delicacy – whitebait! As the mist lifts and the fishing comes to an end for the day, conversations turn to that critical question… what’s the best recipe for whitebait fritters? I don’t claim to have the answer to that, but I do hope to provide some insight into how many eggs might be needed… and the answer might just surprise you!
Whitebait are the juveniles of five species of native fish (see Amber McEwan’s article for more details), returning to freshwater after spending the first few months of their lives at sea. This migration into freshwater is a critical part of their life-cycle, as they move upstream to find the habitats where they will feed, grow into adults, mature and then begin the cycle all over again. It is that last aspect, beginning the life-cycle all over again, that I’m going to focus on here.
Understanding of the spawning biology of the whitebait species is relatively poor and, with the exception of inanga (Galaxias maculatus), is based on only occasional observations. However, what we do now know, after the spawning sites of giant kokopu (Galaxias argenteus) were discovered for the first time last year, is that all five species have a somewhat unusual reproductive strategy. This makes them uniquely susceptible to changes to river flows and bankside vegetation. Mainly in autumn and winter (depending on which species and where you are) all five species have been found to deposit their eggs in habitats only temporarily submerged by high water levels. For inanga, which migrate downstream to estuaries to lay their eggs, this is high spring tides. For the other four species, this is high river flows caused by rainfall. The eggs of inanga are about 1 mm in diameter (the other species have slightly larger eggs) and are typically laid within dense grasses that retain moisture and provide shading. These conditions help the eggs to survive out of water until they are ready to hatch about three to four weeks later. The eggs then hatch when the spawning sites are re-inundated by future high river flows or, in the case of inanga, high spring tides. The larvae are then washed out to sea, where they stay for several months, before finally returning to freshwater as whitebait. It seems easy really, doesn’t it? Well, let’s look at it in a bit more detail…
An average sized adult female inanga will lay around 2-3000 eggs (McDowall, 1984). Studies have shown that on average, only about 11% of eggs survive to hatch (Hickford et al., 2010). Once the eggs hatch and the larvae make it to sea, survival is very low. No data are available specifically for inanga, but mortality of larval fish in the marine environment has been estimated at >98% (Zeldis et al., 2005). On returning to freshwater, investigations have shown that around 30% of whitebait may be caught in the whitebait fishery (Baker and Smith, 2014). An unknown number of these remaining fish then survive to adulthood and successfully spawn (let’s assume 50%, but it has been suggested this is more likely to be less than 20%). If we assume a cup of whitebait (about 500 fish) is used to make a whitebait fritter, we can work out that it actually takes close to 650,000 eggs to make a whitebait fritter!
Baker, C. & Smith, J. 2014. Influence of flow on the migration and capture of juvenile galaxiids in a large river system. New Zealand Journal of Marine and Freshwater Research.
Hickford, M. J. H., Cagnon, M. & Schiel, D. R. 2010. Predation, vegetation and habitat-specific survival of terrestrial eggs of a diadromous fish, Galaxias maculatus (Jenyns, 1842). Journal of Experimental Marine Biology and Ecology, 385, 66-72.
McDowall, R. M. 1984. The New Zealand whitebait book, Wellington, Reed.
Zeldis, J. R., Ooldman, J., Ballara, S. L. & Richards, L. A. 2005. Physical fluxes, pelagic ecosystem structure, and larval fish survival in Hauraki Gulf, New Zealand. Canadian Journal of Fisheries and Aquatic Sciences, 62, 593-610.
Dr Paul Franklin is a Freshwater Ecologist at NIWA.