The growth of cultured Perna canaliculus in Pelorus Sound, New Zealand : the importance of spat origin, environment and time of harvest.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Factors affecting the growth and condition (flesh weight/total weight) of the greenshell® mussel, Perna canaliculus, were investigated in Pelorus Sound, on the northern coast of the South Island of New Zealand. The relative importance of time of harvest, location (two mussel farms in each of three areas), and the geographic origin of mussels (three stocks: Golden Bay, Kaitaia, and the Marlborough Sounds) were measured in four growth trials between August 1997 and October 1999. Particular attention was paid to the condition cycles of mussels because poor condition during winter often results in a severe reduction in yields and/or the cessation of commercial harvesting. If certain mussel stocks, or mussels grown in particular areas of Pelorus Sound, remain in better condition during winter, the selective harvesting of these mussels would potentially increase yields. The condition of mussels declined sharply in mid-winter. This coincided with a rapid decline in the number of mussels with mature gonads and an increase in the number of mussels with immature gonads. This indicates spawning causes the poor condition of farmed mussels in winter. Following winter spawning in both 1998 and 1999 the condition index of mussels at all six study sites declined to very low levels (< c. 30%) regardless of pre-spawning condition. Outside the winter spawning period mussels with high condition indices (c. 40-50%) were nearly always available. Stock had a significant, although small, effect on the condition cycle of mussels. Immediately following winter spawning the condition index of the stock originating from Golden Bay was c. 2-3% higher than the stock from Kaitaia. The period of time that the Golden Bay stock remained in better condition did, however, vary between one and four months in the four growth trials. Because the differences in condition cycles between stocks were small and the length of time that the Golden Bay stock was in better condition varied, it would be difficult to increase the yields from farmed mussels by selectively harvesting stocks at different times of the year. There was also no evidence that growing particular mussel stocks in specific areas of Pelorus Sound could enhance yields. The shell growth rate of Golden Bay stock was c. 25% greater than Marlborough Sounds and Kaitaia stock in the first of the four growth trials. This did not occur in subsequent trials. The difference in growth in the first trial was attributed to the stocks being grown at different locations in Pelorus Sound prior to the trial. If this is the case, it may be feasible to enhance the growth of farmed mussels by manipulating the environment (e.g. location) that spat are exposed to early in life. Because previous studies did not expose mussel stocks to the same environments prior to the experiments or attempt to repeat trials, the stock-related traits they identified may not be predictable or consistent features of the stocks. Spatial and temporal factors (the location and timing of sampling) were the key determinants of mussel condition. The largest range in condition index between sites at a single time was 23% (August 1998), and the largest range between times within a site (Hallam Cove) was 22%. This is in contrast to the largest difference in condition between stocks, of 7% (between Golden Bay and Kaitaia stocks in June 1999). The range in condition (and commercial yields) of mussels between study sites and times was therefore highly variable. Between August 1998 and March 1999 mussels in the middle area of the Sound declined in condition from c. 50% to 30%. This change in condition was not related to any clear annual cycle and suggests the amount of mussels that the Sound (or parts thereof) can sustain may change through time. This is an important point for fisheries managers to consider, as a 300% increase in mussel production has been proposed for Pelorus Sounds region. The rate at which mussels recovered from winter spawning varied between the inner, middle, and outer areas of Pelorus Sound and also between years. Following spawning in 1998, the condition of mussels in the middle area of Pelorus Sound recovered quickly and the commercial harvest rapidly returned to pre-spawning levels. Although the rate at which mussels recovered also varied between areas in 1999, condition recovered more slowly and harvests following winter spawning were lower than in 1998. The conclusion of this study is that although stock has a statistically significant (but small) influence on the condition cycle of farmed P. canaliculus, location and time of harvest are the key determinants of condition and commercial yield. Mussel farmers are therefore advised to locate farms across a broad range of areas in Pelorus Sound. This will allow them to exploit the high degree of spatial variability in mussel condition, to minimise the impact of winter spawning events, and therefore maximise yields throughout the year.