247 
Changes in catch rates and length and age 
at maturity, but not growth, of an estuarine plotosid 
( Cnidoglanis macrocephaius ) after heavy fishing 
Email address for contact author: b.chuwen@murdoch.edu.au 
1 Centre for Fish and Fisheries Research 
School of Biological Sciences and Biotechnology 
Murdoch University, South Street 
Murdoch, Western Australia 6150, Australia 
2 School of Life and Environmental Sciences 
Deakin University 
Princes Highway 
Warrnambool, Victoria 3280, Australia 
Abstract — The hypothesis that heavy 
fishing pressure has led to changes 
in the biological characteristics of the 
estuary cobbler ( Cnidoglanis macro- 
cephaius) was tested in a large sea- 
sonally open estuary in southwestern 
Australia, where this species com- 
pletes its life cycle and is the most 
valuable commercial fish species. 
Comparisons were made between 
seasonal data collected for this plo- 
tosid (eeltail catfish) in Wilson Inlet 
during 2005-08 and those recorded 
with the same fishery-independent 
sampling regime during 1987-89. 
These comparisons show that the 
proportions of larger and older indi- 
viduals and the catch rates in the 
more recent period were far lower, 
i.e., they constituted reductions of 
40% for fish >430 mm total length, 
62% for fish >4 years of age, and 80% 
for catch rate. In addition, total mor- 
tality and fishing-induced mortality 
estimates increased by factors of -2 
and 2.5, respectively. The indications 
that the abundance and proportion 
of older C. macrocephaius declined 
between the two periods are consis- 
tent with the perception of long-term 
commercial fishermen and their shift 
toward using a smaller maximum gill 
net mesh to target this species. The 
sustained heavy fishing pressure on 
C. macrocephaius between 1987-89 
and 2005-08 was accompanied by a 
marked reduction in length and age 
at maturity of this species. The shift 
in probabilistic maturation reaction 
norms toward smaller fish in 2005-08 
and the lack of a conspicuous change 
in growth between the two periods 
indicate that the maturity changes 
were related to fishery-induced evo- 
lution rather than to compensatory 
responses to reduced fish densities. 
Manuscript submitted 5 September 2010. 
Manuscript accepted 4 March 2011. 
Fish. Bull. 109:247-260 (2011). 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Benjamin M. Chuwen (contact author ) 1 
Ian C. Potter 1 
Norman G. Hall 1 
Steeg D. Hoeksema 1 
Laurie J. B. Laurenson 2 
The intense harvesting of a fish stock 
can be accompanied by changes in cer- 
tain life history traits of that stock. 
For example, after heavy exploita- 
tion, Atlantic cod (Gadus morhua) in 
the northwestern Atlantic and had- 
dock ( Melanogrammus aeglefinus) on 
the Scotian Shelf matured at smaller 
sizes and younger ages (Olsen et al., 
2004, 2005; Neuheimer and Taggart, 
2010). Because earlier maturing indi- 
viduals are more likely to reproduce 
before capture than late maturing 
individuals, such changes could have 
been the result of selection for the 
genotype for maturation at a smaller 
size or younger age (Marshall and 
Browman, 2007). It is relevant, how- 
ever, that the results of simulation 
studies indicated that, although the 
harvesting of both immature and 
mature individuals and also only 
immature individuals of a stock leads 
to selection for a reduction in length 
and age at maturity, the restriction of 
harvesting to only mature individuals 
produces the reverse effect (Law and 
Grey, 1989; Heino, 1998; Ernande et 
al., 2004). 
Laboratory studies have shown 
that extreme selective harvesting of 
the largest individuals of Atlantic 
silverside ( Menidia menidia) over 
four generations led to a reduction 
in growth rate (Conover and Munch, 
2002) and that this trend was re- 
versed over five generations when 
harvesting was relaxed (Conover 
et al., 2009). The above reduction 
in growth was attributed to the 
selection of individuals with geno- 
types for slower growth, whereas 
the subsequent increase in growth 
was related to the removal of those 
strong selection pressures. Although 
the above experiments are important 
in that they reveal that selection 
can lead to changes in growth, such 
researchers as Hilborn and Minte- 
Vera (2008) and Brown et al. (2008) 
considered that they introduced more 
extreme selection pressures in their 
experiments than are likely to be 
encountered by wild populations and 
did not take into account the type 
of density-dependent effects that 
might be experienced by such 
populations. On the basis of a meta- 
analysis of 73 commercially fished 
stocks of marine species, Hilborn 
and Minte-Vera (2008) concluded 
that there was no conspicuous 
overall trend for growth to either 
increase or decrease in relation 
to fishing intensity. Furthermore, 
their simulation studies indicated 
that the selectivity patterns of 
most commercial fisheries would 
be unlikely to result in substantial 
evolutionary changes in growth. 
