Effect of a Change in Fishing Regulations on the Survival and Capture Probabilities of Rainbow Trout in the Upper Rangitikei River, New Zealand

Abstract

To evaluate effectiveness of a 550-mm (total length) upper size limit introduced because of concerns about the effect of increasing angling pressure, we estimated survival and capture probabilities of rainbow trout Oncorhynchus mykiss tagged in the headwaters of the Rangitikei River between March 1993 and April 1999. Estimates were obtained using simultaneous analysis of recapture data collected during tagging operations and from tag returns by anglers. We found strong evidence that fish greater than 550 mm were released at a higher rate following the introduction of the upper size limit in 1995. There was no evidence that the release rate changed during the study for fish less than 550 mm. The probability that a fish was caught by an angler during the 6-month summer period was higher than the corresponding winter period, but otherwise there appeared to be little variation through time. Neither capture nor release probabilities appeared to depend on fish sex. Larger trout had higher survival probabilities, and this relationship strengthened following the introduction of the upper size limit. Our study provides evidence that the upper size limit regulation was effective in changing both the behavior of anglers and population dynamics of the fish. This is reinforced by length-frequency data showing an increase in the average length of fish following introduction of the size-limit regulation.