Conservation and management in variable environments: the surf clam, donax deltoides

Author

Sue Murray-Jones, University of Wollongong

Year

1999

Degree Name

Doctor of Philosophy

Department

Department of Biological Sciences

Recommended Citation

Murray-Jones, Sue, Conservation and management in variable environments: the surf clam, donax deltoides, Doctor of Philosophy thesis, Department of Biological Sciences, University of Wollongong, 1999. https://ro.uow.edu.au/theses/1041

Abstract

The population ecology of sandy beaches has been largely ignored in comparison to rocky shores, even though sandy beach fauna may be abundant, and of considerable economic and ecological significance. I used ecological and genetic techniques to investigate aspects of the ecology, life history and levels of exploitation of the surf clam or pipi, Donax deltoides .

My data, obtained from sampling pipis from five beaches over five years, highlight spatio-temporal variability in the distribution and abundance of Donax deltoides across all temporal and spatial scales. Within site variability was usually as high as between site or between time variability. Densities could vary by several orders of magnitude over spatial scales as small as 10-20 m. At least some of this variability can be attributed to the fact that Donax deltoides is highly mobile. Size classes were commonly missing from samples, with both medium and larger sized animals effectively missing on 27% of sampling days, although smaller animals were present in 95% of samples. This is a reflection of mobility rather than mortality, as size classes appeared or disappeared at very short temporal scales (days). Different size classes often occupied different heights on the shore, and the zone occupied by the different size classes varied over time. While small and medium pipis were most likely to be located in the swash (in 72% and 48% of samples respectively), large pipis were equally likely to be located in the swash (32%), intertidal (38%) or subtidal zone (30%). The distribution pattern of different size groups was not consistent across short temporal scales (days) or across spatial scales of a few kilometres. This study underlines the inadequacy of the single transect approach adopted many workers.

For smaller animals initial growth was rapid, from 6 mm to 30 mm within 4-8 months, and growth was asymptotic. Growth rates were fairly consistent both between and within regions. Pipis appeared to reach 37 mm in 37 mm insites, at which size 50% were sexually mature. Growth clearly slowed with size. Mortality varied both between and within sites, with few large animals found at some sites. Recruitment patterns were consistent both between and within sites.

Reproductive data, derived from a variety of sources including gonad smears, condition indices, oocyte diameters and stereology, revealed that the spawning pattern of Donax deltoides was poorly defined. I found considerable variation in the timing of reproduction from year to year, both among individuals and among years. The size at which pipis can be defined as fully mature was consistent both between sites and among years. Gonad smears indicated that at least 50% of pipis were mature by 37 mm in length, with gametes being found in animals as small as 27 mm. Populations at two widely separated locations showed prolonged spawning, with some peaks in activity. At both sites, mature females (≥37 mm) contained apparently mature eggs nearly all year. The number of eggs held per female was low, but overall fecundity is likely to be high as females released eggs over a long period.

A consequence of such a prolonged spawning period was the presence of recruits beaches all year round, however juvenile mortality was high and large numbers of recruits did not go on to establish detectable cohorts. Cohorts only established between July and December during this study at all sites. I found consistent recruitment even on beaches where there were very few adults, indicating that there was not a strong stockrecruitment relationship in this species.

This finding was supported by electrophoretic data, suggesting that local populations of pipis on the east coast of Australia are strongly connected and form a single fishery. I surveyed patterns of allelic variation within 12 samples of pipis from beaches separated by up to 1200 km along the east Australian coast, influenced to varying degrees by the East Australian Current. On the east coast of Australia the erratic nature of this current, the formation of closed rip cells which limit the circulation of water offshore from surf beaches, and the turbulent nature of the surf zone itself (which may lower fertilisation success) might be expected to restrict larval dispersal and make the supply of recruits variable. However I found no evidence of population subdivision, with little genetic variation among all samples (F_ST = 0.009), among geographic regions (F_RT = 0.001), or among samples (F_SR = 0.010). These data imply that larvae are moving between regions and that levels of present or recent gene flow are high. I found no accumulation of rare alleles at either extreme of the distribution studied, which suggests that gene flow is bidirectional.

I found large differences between harvesting patterns in the recreational and commercial pipi fisheries, implying that the sectors may have very different impacts on local pipi stocks, even though both fisheries were restricted solely to hand gathering. I estimated the catch from Stockton Beach on the mid-north coast, which accounts for half of the total catch of pipis in NSW. I found a combined recreational and commercial catch of pipis of 237.7 tonnes during the period March 1996 to February 1997 inclusive, taken in a total of 120,672 collector hours. The commercial fishery was characterised by a relatively large catch, low fishing effort, and high catch rate. In contrast, the recreational fishery was characterised by a relatively small catch, high fishing effort, and very low catch rates. Recreational fishers took 20% of the combined commercial and recreational catch on Stockton Beach, but accounted for 89% of the fishing effort, with an estimated 15,795 parties participating. The majority of recreational harvesters were collecting for food, rather than bait. I also compared the recreational harvest at Stockton Beach with that of Seven Mile Beach on the south coast of NSW. I found clear differences in the recreational catch between the north and south coasts of NSW, with extremely low catch rates in the south.

In general my data fit with life history predictions for an outcrossing planktotrophic species in a variable environment. Pipis became sexually mature at an early age, produced large numbers of small eggs, and had low but variable reproductive effort. Year round spawning may mitigate the effects of a stochastic environment, particularly if spawning in Donax deltoides is in response to oceanographic triggers. Prolonged partial spawning may be a form of bet-hedging in exposed environments with changing environmental conditions.