Nonnative Fishes of the
Cape Fear River System
blue catfish (Ictalurus furcatus) was
introduced to the Cape Fear River by the Wildlife Resources Commission in the
attempt to create a trophy fishery (Moser and Roberts 1998).
Although blue catfish were uncommon in the 1970's, they are currently the
most abundant species captured in our gillnet survey (Mallin et. al.
1998,1999,2000). The success of the
blue catfish in the Cape Fear River system is likely due to it's generalist
feeding behavior. Gut content
analyses have shown this species to feed on a wide range of prey including
snakes, birds, fish, shrimp, worms, eels, grapes, other fish and surprisingly
clams. Over 75% of the stomachs
examined contained an Asian fresh water clam (Corbicula
fluminia) that was introduced by a bilge discharge in the Wilmington harbor
in 1975 (Williams and Moser, in prep).
Although thought to have aided in the demise of our native catfish
population through competition, blue catfish are a popular sport fish and
support a small commercial fishery in the Cape Fear River. Disease
percentages ranged from 28% in the fall of 1999 to less than 1% in the spring of
2001 (Mallin et all 2000). Although
there have been no significant changes in catch-per-unit-effort (Figure 26),
this species will be monitored closely for information on the cause of the
disease percentage fluctuations.
1966 the North Carolina Wildlife Resources Commission introduced the flathead
catfish (Pylodictis olivaris) to the
Cape Fear River in an attempt to create a trophy fishery (Moser and Roberts
1998). Within 15 years of
their introduction, the flathead catfish was found to be the most abundant
catfish by weight and considered to be the new dominant predator in the Cape
Fear (Guier et. al. 1981). Guier's
study in the late 1970's showed that fish (99.4% by weight ) were the principle
prey of P. olivaris. Catfishes
were the dominant fish found in the flathead's diet (Guier et al. 1981, Ashley
et al. 1989). This is a strong
indication that the introduction of this species has led to the severe decline
of our native catfish populations. Since 1997 only 2 native catfish have been
captured while 1618 blue catfish, 193 channel catfish, and 198 flathead catfish
have been captured. Thus less than
0.1% of our catfish captures are native species.
Future studies should reexamine the diet of flatheads to determine which
prey species are currently being exploited as a food source.
Flathead catfish exhibited a statistically significant decrease in the
fall 2000 but exhibited a statistically significant increase in spring 2001
gillnet catch-per-unit-effort (Figure 30).
Reasons for these changes are yet unknown but will be addressed in future
Channel catfish were introduced into the Cape Fear in the early 1900's (Smith 1907). A small but stable population was established that persisted through the 1970's. In recent years, however, this species has shown "reductions in relative abundance since the introduction of the blue and flathead catfishes."(Moser and Roberts 1998). The decline is likely due to competition with blue catfish. This survey shows no statistically significant changes in catch-per-unit-effort (Figure 28) and a low incidence of disease since 1997.
carp (Ctenopharyngodon idella) are
another non-native species of concern. Grass
carp have reached sizes of over 60 pounds in this state.
They are herbivores and have been introduced to reservoirs and ponds
throughout the Cape Fear River basin to control aquatic vegetation.
When they are introduced to the Cape Fear by flooding events, however,
they consume aquatic vegetation that functions in controlling erosion and as
nursery habitat for juvenile fishes. The
state of North Carolina recognized the potentially destructive habits of this
species and requires that all grass carp be certified as triploid before they
can be introduced to ponds and reservoirs. A recent study in the Chesapeake Bay
found that although stocking of non-sterile grass carp has been illegal since
1979, 18% of the feral grass carp collected in Chesapeake bay tributaries were
not triploid. The researchers
speculated that the non-triploid carp originated from illegal stocking efforts
or had been introduced them before the regulations were put into place (Schultz
et. al. 2001). If a mistake has
been made and 100% of the grass carp introduced were not sterile, then there
could be a reproducing population of grass carp in the Cape Fear. If conditions
are favorable, it takes only a few individuals to populate a river system.
An example would be the flathead catfish.
Eleven individuals were introduced in 1966 and they are now one of the
dominant predators in the Cape Fear River system.
A reproducing population of non-native grass carp could thus severely
impact our fisheries resources (Raibley et al. 1995).
There was no statistically significant difference in
catch-per-unit-effort between years. Although
more were captured in the fall of 1999 than all previous years combined, there
is a trend toward decreasing catches of grass carp since that time.
Although the documented trend is encouraging, monitoring of this species
should remain a priority of this survey to examine changes in population levels
and determine if they indicate reproduction in this river system.
sunfish (Lepomis macrochiris) are
another introduced species in the Cape Fear.
In this survey they are the second most abundant sunfish captured after
bluegill. They are an important pert of the forage base and support a popular
recreational fishery. Although not statistically significant, there has been a
trend toward increased catch-per-unit-effort in the spring electroshocking
samples (Figure 36). Disease
percentage average continues to average approximately 4 percent with no