Factors limiting resilience and recovery of fished abalone populations
Mundy, CN and Miller, KJ, Factors limiting resilience and recovery of fished abalone populations, Factors limiting resilience and recovery of fished abalone populations, IMAS/Sf CRC, Hobart, Tasmania, 1, pp. 1-127. (2012) [Government or Industry Research]
Copyright 2010 Australian Seafood CRC and the Tasmanian Aquaculture and Fisheries Institute, University of Tasmania
Recovery of depleted abalone populations to a productive level will be dependent on
recruitment success occurring over multiple years. High levels of exploitation or,
natural mortality events, can reduce the reproductive capacity of populations to levels
where local populations are no longer self-sustaining. Translocation of mature adult
abalone to create standalone spawning populations have been proposed by managers,
industry and researchers in a range of abalone fisheries around the world, although this
concept has never been fully tested. In this study we examined translocation of wild abalone to depleted abalone reefs in North-East Tasmania as a tool to enhance the rate
of recovery of reefs that have failed to recover naturally over several decades.
Several experiments were conducted during this study, including a wild abalone
translocation, a genetic study on connectivity, monitoring of larval recruitment at
Treatment and Control sites, and a larval tagging study.
The translocation experiment involved moving around 2000 wild abalone to each of
three Treatment sites at an initial abalone density of 8/m2. Surveys of the three
Translocation sites and three neighboring Control sites were conducted over a 24 month
period, clearly showed an initial effect of the translocation with significant shifts in
density. Density of larger abalone gradually declined over the subsequent 24 months as
a result of natural processes, notably emigration at two of the Treatment sites, and
mortality from a single severe storm event at the other. While the study time-frame was
too short to determine whether the translocated wild abalone did indeed function as a
viable spawning population, we were able to determine that translocation of wild
abalone can be done with little cost, and with high initial survival.
The population genetic study confirmed results from previous genetic and field studies
conducted in Tasmania, that dispersal of abalone larvae is limited, with most
populations largely reliant on self-recruitment. This result has very clear implications
for the scale of benefit that might be achieved from translocation of wild abalone, with
the benefits largely restricted to the natal site (i.e. site of release). Thus to rebuild
populations over a large geographic scale, a large number of release sites will be
required. This finding applies equally to stock enhancement using hatchery raised larval
or juvenile abalone.
In the context of management of wild abalone fisheries, maintenance of commercially
viable densities of abalone on exploited reefs will also be dependent on the local
reproductive biomass. Thus appropriate Minimum Legal Sizes (MLS) to preserve
sufficient reproductive biomass, matched with appropriate Total Allowable Catch
(TAC) is fundamental to ensuring that our abalone fisheries are resilient to ongoing
Recruitment of abalone larvae to artificial collectors was highly variable, with the
majority of recruitment to collectors occurring at just a single site. At the time when spawning must of occurred, the site at which recruitment was observed was also the site
with the highest density of abalone. This result is consistent with the concept of a
threshold density, or Allee Effect, above which successful recruitment can occur.
However, it was not possible to determine whether the limiting factor was suitable
habitat or supply of larvae. Processing of larval collector samples is very slow, and
requires high levels of human resources. We suggest some tactical research be
undertaken in order to improve the efficiency of sample processing to make future
recruitment studies feasible.
Experiments to identify suitable methods for chemical tagging of larvae were partially
successful. Nile Red was the most successful tag, and acts by creating a fluorescent
mark on lipid reserves of the egg or developing embryo. Nile Red had little impact on
normal development of embryos, or settlement. However, the intensity of the
fluorescent tag diminished rapidly at metamorphosis, and was therefore unlikely to be
useful for the intended purpose, which was to distinguish larvae released from known
points from larvae that may have arrived from elsewhere.