The population size of amphidromous fish species is limited by three potential bottlenecks, i) adult carrying capacity, ii) egg carrying capacity and iii) recruitment limitation. For the five species of amphidromous galaxiids that make up the whitebait fishery in New Zealand there has been a recent focus on tightening fishing regulations, with the goal of reducing recruitment limitation by increasing the number of juveniles that make it into rivers.
Here, we present a metapopulation model representing the life cycle of an amphidromous fish. We then use the model to explore how combinations of variable adult carrying capacity, egg carrying capacity and recruitment limitation affect metapopulation size, using Galaxias maculatus as a case study.
We show that a high level of recruitment limitation is required to affect metapopulation dynamics and abundance, but once a threshold is passed populations may collapse, conversely the metapopulation responds linearly to changes in adult and egg carrying capacities.
Our results suggest in all situations, except the most extreme cases of recruitment limitation, increasing adult and egg carrying capacities (e.g. through habitat restoration) is more likely to have a meaningful effect on increasing population size, compared to focusing on factors affecting recruitment limitation, such as whitebaiting.