Chuwen et al.: Changes in the catch rates and length and age at maturity of Cmdoglanis macrocephalus 
257 
Discussion 
The fishery in Wilson Inlet is a multisector, i.e., com- 
mercial and, to a lesser degree, recreational, multigear, 
and multispecies fishery, of which the benthic C. mac- 
rocephalus is just one of several species targeted during 
commercial fishing. The data recorded by commercial 
fishermen for the managers of this fishery are greatly 
influenced by the variable extent to which the different 
species are targeted overall and during different periods 
and do not identify the fishing effort directed toward any 
single species. Such fishery-dependent data thus provide 
little information on the status of the C. macrocephalus 
stock in Wilson Inlet and how it might be changing over 
time. At present, such information can only be obtained 
from the type of fishery-independent data that were 
collected during the present study. The confinement of 
the stock of C. macrocephalus in Wilson Inlet to that 
estuary, and the availability of sound biological data 
for this stock in the 1980s, have provided a particularly 
good opportunity to explore the effects of heavy fishing 
on the biological characteristics of an exploited fish 
population. In particular, the existence of these earlier 
biological data allowed us to investigate the possibility 
that length or age at maturity, or both, changed as a 
result of fishing-induced evolution. 
The fact that the mean catch rates of C. macrocephalus 
in Wilson Inlet during 2006-07 were 80-85% less than 
those in 1988 indicates that the abundance of this 
plotosid in this estuary declined markedly between 
those two periods. It is thus highly relevant that the 
samples during 1987-89 were dominated by age 3+ and 
to a lesser extent age 2+ fish and contained appreciable 
numbers of fish >4 years old and some >7 years old, 
whereas those in 2005-08 were dominated by the 
2+ age class and contained very few fish older than 
3 years. The smaller proportion of older fish during 
2005-08 is reflected in the smaller percentage of fish 
caught with lengths greater than the current MLL of 
430 mm TL during this more recent period, i.e., 29% vs 
48%. The conclusion, based on fishery-independent data, 
that the numbers of large fish have declined during 
the last two decades, is consistent with experienced, 
long-term commercial fishermen responding to a 
reduction in their catches of larger fish by using a 
smaller mesh in their gill nets (McIntosh 2 ; Miller 3 ). 
Because the decline in catch rates between 1987-89 
and 2005-08 was related to the marked reduction in 
the proportion of C. macrocephalus with lengths greater 
than the MLL for retention of this species, there is very 
strong circumstantial support for the conclusion that 
this decline was related to the effects of fishing rather 
than to a change in the environment. 
The decline in the relative abundance of larger 
and older fish in Wilson Inlet between the two 
periods accounts for the estimates of total mortality, 
Z, increasing markedly from 0.92 per year during 
1987-88 to 1.82 per year in 2006-08. Furthermore, 
fishing mortality, F, increased from 0.57 to 1.47 per 
year between those two periods, which represented an 
increase of more than 250%. Although F in the earlier 
period was already about 1.6 times that of natural 
mortality, M, it had become more than four times greater 
than that of M by 2006-08. The current level of F for 
this species in Wilson Inlet is therefore well in excess 
of 0.75 or 0.8 M, which have been considered target 
reference points for F in data-poor fisheries (Gabriel and 
Mace, 1999), and is thus at a level unlikely to sustain 
the fishery. The view that commercial fishing has led 
