VIABILITY OF ABALONE (HALIOTIS IRIS) STOCK ENHANCEMENT BY RELEASE OF HATCHERY-REARED SEED IN MARLBOROUGH, NEW ZEALAND

Abstract

The total allowable commercial catch from New Zealand's Haliotis iris Gmelin 1791 fishery was reduced by 18% between 1999 and 2004. Quota holders have initiated research to assess the viability of stock enhancement by release of hatchery- reared seed. Boulder reefs (1 3 2 3 0.5 m) were constructed by placing natural boulders in wire baskets over sand or bedrock. These reefs allowed accurate census of small abalone during short-term experiments (3-4.5 mo.) examining the effects of seed size (5-25 mm shell length (SL)) and density (25-640 m -2 of seafloor) on survival and growth. Survival increased with seed size, but beyond 10 mm the increased survival did not offset the cost of larger seed. Growth and survival of 8-24 mm seed decreased with increasing density, but regressions were nonsignificant (P ¼ 0.06-0.36) because of variability among reefs. Densities as high as 300 m -2 gave good growth and survival (>40%) over 3 mo. on some reefs. Five natural sites were seeded with 2,600-20,000 juveniles (mean 10-11 mm SL, range 6-19 mm) at an average density of 50 m -2 to estimate long-term survival from commercial reseeding. After 17-20 mo, when recovered abalone averaged 47-60 mm SL, survival varied widely among the five sites ranging from 1.7% to 25.1% (average 13.8%). Estimated survival to harvest size of 125 mm SL ranged from 1.3% to 18.6% (average 10.2%) assuming 3 further years of mortality at M ¼ 0.1. The two sites with the lowest survival were affected by substrate movement during storms, highlighting the risk of using exposed locations with boulders small enough to be turned for surveys. Survival to harvest averaged 15.2% across the three sites without significant storm damage. Growth averaged 29.5 mm SL year -1 across the five sites (range 25-33 mmyear -1 ). A model was used to examine the economic viability of reseeding, assuming that reseeded abalone supplement natural recruits. At a price of NZ$0.32 per 10 mm SL seed, the return on investment was 20% yr -1 at 10% survival to harvest, and 30% yr -1 at 15% survival. These returns compare favorably with opportunity costs of ;10% yr -1 , suggesting that reseeding is likely to be economically viable if sites and habitat are carefully selected. Large scale seeding should be accompanied by monitoring to quantify net population increase.