438 
Abstract .—The blacklip abalone, 
Haliotis rubra, fishery is the most com- 
mercially important in Victoria, Austra- 
lia. Two underwater census methods 
have been used to survey the status of 
its stocks. Collection rates from timed 
searches and counts from radial tran- 
sects were used to estimate abalone 
abundance at various locations along 
the Victoria coast. Radial transect sur- 
veys provided precise estimates of aba- 
lone abundance; these did not vary sig- 
nificantly between research divers un- 
der controlled conditions. During stock 
monitoring, there were no significant 
differences among nine of twelve re- 
search divers’ estimates of abalone 
abundance from 1339 radial transects 
sampled at 60 sites during 1992-94. 
Monte Carlo simulations, used to esti- 
mate the probability of detecting an- 
nual changes in abalone abundance 
with each method, provided evidence 
that radial transects have the poten- 
tial to detect smaller annual changes 
in abalone abundance than do timed 
searches. We suggest that radial tran- 
sects provide a credible alternative to 
timed searches and are less subject to 
potential biases that affect the latter. 
Manuscript accepted 20 October 1997. 
Fishery Bulletin 96:438-450 (1998). 
A comparison of two underwater 
census methods for estimating 
the abundance of the commercially 
important bfacklip abalone, 
Haliotis rubra 
Harry K. Gorfine 
David A. Forbes 
Anne S. Gason 
Marine and Freshwater Resources Institute 
RO. Box 1 14, QueensclifT, Victoria, Australia 3225 
E-mail address (for H.K. Gorfine): H.Gorfine@mafri.com.au 
Most analyses of fish stocks are 
based on catch per unit of effort 
(CPUE) as a measure of abundance. 
There appears, however, to be con- 
sensus among investigators of aba- 
lone that fisheries-dependent meth- 
ods are inadequate for the estima- 
tion of the abundance of abalone 
stocks. Reasons for this are well 
documented (Breen, 1992; McShane 
and Smith, 1992; Prince and Guz- 
man del Proo, 1993) and include 
difficulties in estimating the true 
catch (due in no small part to the 
prevalence of illegal harvesting) and 
hyperstability in catch rates despite 
stock reduction (Hilborn and Wal- 
ters, 1992). The latter stems from 
the highly efficient searching be- 
havior of abalone divers who, by 
targeting large aggregations of aba- 
lone, may serially deplete substocks 
(Prince and Guzman del Proo, 1993). 
Victoria, Australia’s second larg- 
est abalone-producing state, yields 
a total annual quota of 1440 metric 
tons (t) of abalone from approxi- 
mately 121 km 2 of commercially 
productive reef complexes distrib- 
uted along approximately 1500 km 
of coast (McShane et al., 1986). The 
Victorian abalone fishing grounds 
are subdivided into central, eastern, 
and western zones (Fig. 1), each 
with its own total allowable catch 
(TAC ). Abundance estimation meth- 
ods used to survey such an exten- 
sive distribution of abalone stocks 
must be simple so that they may be 
applied under often adverse sea con- 
ditions and so that several sites may 
be sampled per day. 
A reliable survey method for the 
estimation of abalone abundance 
has yet to be determined although 
several diving-based methods are 
currently used to assess the status 
of stocks in southern Australia and 
New Zealand. These include timed 
searches (McShane, 1994), transect 
surveys, patch-size estimates (Mc- 
Shane, 1996), change-in-ratio (Nash 
et al., 1994), and Petersen mark- 
recapture surveys (Hart and Gor- 
fine, 1997) to provide measures of 
relative abundance. The intention 
of such surveys is to provide trends 
in relative abundance necessary for 
assessment of stock status. These 
methods have the advantage of be- 
ing fishery independent. 
However, underwater census 
methods are characterized by varia- 
tion between research divers. In 
particular, relative abundance esti- 
mates from timed searches must be 
standardized between research 
divers to achieve uniformity in the 
base sampling unit (10 minutes). 
Standardization of many research 
