A Preliminary Assessment of the Abundance, Size and Population Genetic Structure of Pipis
RECREATIONAL FISHING GRANTS PROGRAM RESEARCH REPORT: AUGUST 2011
The collection of pipis (Donax deltoides) has been practised along the Australian coast for over 10,000 years by indigenous Australians. Following European settlement the harvest has continued by both commercial and amateur fishers. Pipis exhibit highly variable recruitment and abundances, and are easily extracted from the swash zone on ocean beaches. These attributes have led to this species becoming susceptible to over-exploitation in many regions as illustrated by population declines in South Australia, New South Wales, and Victoria.
In Victoria, pipi stocks in the Venus Bay area have experienced a dramatic decline prompting the implementation of reduced catch limits and stock assessment research by fisheries managers. Similarly, evidence of stock declines has also been detected in Discovery Bay in south western Victoria. This region has a long history of amateur pipi collection and as of 2011 this area is now supporting a commercial fishery. Anecdotal evidence suggests a reduction in abundance and average size has occurred during the last decade highlighting the need for stock assessments in the area.
The purpose of this report is to provide (1) a baseline assessment of abundance, biomass and size structure at three sites in the Discovery Bay Coastal Park using methods that mimic collection by recreational fishers; and (2) conduct a preliminary assessment of genetic stock structure in south eastern Australia using mitochondrial DNA.
Distribution, size and abundance
Sampling of Discovery Bay Coastal Park occurred on the 28th of October 2010, the 22nd of December 2010 and the 28thb of January 2011 at three locations (Nelson; Nobles Rocks; Lake Monibeong). Two methods were used to assess the abundance of pipis at each site. Juvenile abundances and biomass were assessed using a corer. Abundance of larger pipis that are typically targeted by recreational fishers was assessed using timed searches that were designed to mimic recreational fishing effort. Replicate 5-minute collections were taken at each location according to Victorian recreational fishing regulations (i.e. hand collection only without nets or rakes). Pipis collected using both methods were counted and weighed on site (wet weight). Juveniles were found to vary significantly through space and time, with greater abundances at all three locations observed during the October sampling period. The total number of juveniles collected from the three locations during the three sampling times was 6,480, having a total wet biomass of 7.13 kg (n = 405 cores). Juvenile abundances were greatest at Nobles Rocks on all three occasions. The trend for higher abundance at Nobles Rocks was also evident for larger individuals, with the exception of the December collection period when biomass and numbers were higher at Lake Monibeong. The total number of adults collected across all sampling periods was 3016 individuals, which weighed 33.45 kg during a combined search time of 20.25 hrs. The number of pipis collected during a single 5 min search ranged from 0 - 47 at Lake Monibeong, 3 - 78 at Nobles Rocks, and 0 - 18 at Nelson. Timed searches revealed that recreational fishers have a high likelihood of finding at least 1 pipi during a single 5-minute search, with only a small number of searches returning a zero count (n = 5 of 243 timed searches).
GENETIC STOCK ASSESSMENT
Pipis from the Coorong (South Australia, n = 11), Nelson (Victoria, n = 12), Lake Monibeong (Victoria, n = 9), Venus Bay (Victoria, n = 13), Golden Beach (Victoria, n = 5) and Eden (New South Wales, n = 11) were examined for genetic variation at the mitochondrial cytochrome oxidase subunit I gene. A total of 18 unique mitochondrial haplotypes were identified (frequency ranging from 0.08 - 0.80 among sampling locales) with a single dominant haplotype distributed broadly across the sampling area. Genetic analyses indicated no evidence of population differentiation among the six sample sites suggesting historical population connectivity in south eastern Australia.
Prior to this study therewas no available information on the abundance, biomass or size of pipis in Discovery Bay. Therefore, results of the present study providea baseline assessment for this region for future comparison. This is of particular importance because commercial harvesting has recently commenced in south west Victoria, including Discovery Bay, in addition to the traditional recreational fishing pressure. Due to the variable nature of pipi distributions and abundance, it is difficult to compare between locations, however longer-term monitoring at Discovery Bay is encouraged and will allow fishery managers to make better informed decisions to ensure sustainable use of this resource.
The genetic stock structure of pipis in south eastern Australia remains largely unresolved. Genetic stock structure is currently being investigated further in a joint collaboration between Deakin University and The University of Melbourne. This additional research will involve a more comprehensive genetic analysis using multiple independent genetic markers (nuclear microsatellites), increased sampling and greater geographical coverage of samples spanning South Australia to Queensland. This study will identify contemporary patterns of gene flow and genetic stock structure and will provide a valuable spatial framework for managers working across wider geographical areas.
Versace, V. L.a, Matthews, T. G.b, Miller, A. D.c, Bowie, K.C.b, Ierodiaconou, D.b, Mills, D.b & Lewis, Z.d
a School of Information Systems, Deakin University
b School of Life and Environmental Sciences, Deakin University
c Department of Zoology, University of Melbourne
d Fisheries Victoria
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