Comparative ecology and interspecific competition were
examined between two sympatric congeners, Sebastes caurinus
Richardson 1845 (copper rockfish) and S. maliger (Jordan and
Gilbert 1880) (quillback rockfish) in Saanich Inlet, British
Columbia, Canada, from 1986-1990. Ecological profiles were
constructed through analyses of depth distribution, habitat
and species associations, activities, feeding habits, gut
allometry, growth, and reproduction. Interspecific
competition between copper and quillback rockfish was
examined by experimentally manipulating the densities of one
or the other species on rocky reefs in Saanich Inlet where
they were sympatric.
The Pisces IV submersible was used to survey the
distribution of rockfish in relatively deep-water (21-140 m)
in Saanich Inlet. Copper and quillback rockfish were
sympatric in water depths of 21-65 m. They occurred in
association with one another the majority of the time (>90%)
and their densities were greatest over areas of complex
substrate. Size of copper and quillback rockfish was
positively correlated with increased depth, primarily due to
the absence of small fish in deeper waters. Both species
were observed most frequently perched on open substrate or
hovering in the water column. Copper rockfish were observed
swimming more frequently than quillback rockfish.
Copper and quillback rockfish primarily consumed
demersal crustaceans throughout the year. Copper rockfish
consumed a greater proportion of pelagic fishes than
quillback rockfish, whereas quillback rockfish had a greater
proportion of pelagic crustaceans in their diet. Levins'
(1968) measure of niche breadth of the diet (by mass), as
standardized by Hurlbert (1978), was narrow (0.19-0.20) to
moderate (0.32-0.51) for quillback and copper rockfish
respectively, during spring, summer, and fall. In the
winter it was extremely narrow (0.02) for both species due
to their feeding predominantly on one prey type, juvenile
herring (Clupea harengus) . The Simplified Morisita Index of
niche overlap (Horn 1966) in feeding habits (by mass) was
relatively high (>0.55) throughout the year, and
particularly during the winter (0.99). This high niche
overlap in the winter occurred when large schools of
juvenile herring were available in the environment and were
probably not a limited resource. Extensive niche overlap
between copper and quillback rockfish may therefore indicate
an abundance of a shared resource rather than competition
for the resource.
Copper and quillback rockfish consumed the greatest
quantity of food during the winter when feeding on juvenile
herring, although quillback rockfish consumed significantly
less food mass than copper rockfish in the winter. A
greater proportion of quillback rockfish were collected with
food in their stomachs during the spring and summer, when
the numerically dominant food items were pelagic
crustaceans. The importance of fish prey in the diets of
both copper and quillback rockfish increased with size.
Copper rockfish had a shorter intestine and larger
stomach relative to similar-sized quillback rockfish. This
suggested that the gastrointestinal tract of copper rockfish
was better suited to holding and digesting fish and larger
crustaceans than quillback rockfish, an observation
consistent with differences in their feeding habits.
Copper and quillback rockfish had similar growth
patterns with no readily identifiable species-specific and
sex-specific differences. Both sexes of both species
attained asymptotic lengths of 30-31 cm total length and had
similar growth coefficients (0.141-0.187). Within each sex,
copper rockfish had a smaller increase in mass per unit of
body length than quillback rockfish, indicative of a more
pelagic lifestyle for copper rockfish.
Estimated lengths at first and 100% sexual maturity for
female and male copper and quillback rockfish were similar.
Male copper rockfish were ripe, and potentially inseminated
females, in January and February. Female copper rockfish
were found to be carrying fertilized eggs in April and May,
and gave birth to their young primarily in June. The
reproductive cycle of quillback rockfish preceded that of
copper rockfish by approximately one month, with parturition
for quillback rockfish occurring mainly in May. The
fecundity of copper and quillback rockfish was similar, with
a 30-cm fish giving birth to approximately 90,000 young.
Visceral fat cycles of mature female copper and
quillback rockfish were complementary to their cycles of
gonad maturation and increases in gonad size, indicating
that they use visceral fat stores as a source of energy for
maturation of their eggs and nourishment of their developing
young. Visceral fat cycles of mature males were mainly
coincident with the maturation and size increase of their
gonads, indicating that they did not use visceral fat
reserves in the maturation of their gonads. Male rockfish
secondarily may have used their fat reserves as an energy
source during the period when they were ripe, perhaps for
mating activities. Visceral fat accumulation and
dissipation in immature males and females appeared to be
primarily related to periods of feeding.
Interspecific competition between copper and quillback
rockfish was asymmetrical, seasonal, and transitory, based
on experimental manipulations of the densities of the
congeners in natural populations. Copper rockfish did not
have a competitive effect on quillback rockfish, but
quillback rockfish had a weak competitive effect on copper
rockfish. This effect was apparent only during the fall,
was strongest in the fall immediately following the density
manipulations, and appeared to weaken in the subsequent fall
season. The seasonal competitive effect may have been
caused by copper rockfish moving onto the study reefs (18-31
m depth) from shallower waters (<20 m) during the fall and
winter, creating a short-term 'ecological crunch' in which
food or space resources were limited.
Overall, comparative ecological profiles of copper and
quillback rockfish exhibited a large degree of overlap.
Differences observed between them were small but
consistently indicated that copper rockfish had a more
pelagic lifestyle than quillback rockfish. The otherwise
high degree of similarity between the two congeners,
however, did not translate into sustained interspecific
competition.
Ecological theory purporting a major role for
interspecific competition in structuring fish communities
was therefore not supported by experimental manipulations of
population densities of deep-subtidal, temperate zone
rockfishes. The asymmetrical, seasonal, and transitory
occurrence of weak interspecific competition demonstrated
that competition between these rockfish species is dynamic,
and cannot account for the pattern of species association.
Alternative hypotheses based on the importance of
intraspecific competition, predation, or environmental
variability must therefore be considered. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9560 |
| Date | 29 June 2018 |
| Creators | Murie, D. J. |
| Contributors | Mclnerney, J.E. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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