Science: Hybridization among Asian carp may reduce impacts of future invasions

Big head carp and silver carp (also known together as Asian carp, the fishes currently threatening to invade Lake Michigan) have been observed to interbreed, with future generations of hybrids exhibiting reduced growth and fitness.  Lamer et al. (2010) conducted a study to see if big head carp Hypopthalmichthys nobilis and silver carp H. molitrix are hybridizing in the Mississippi and Illinois Rivers, which species or sex is more likely to interbreed and if commonly used physical traits accurately determine an individual’s taxonomic identity.  With trammel nets, 120 carp were collected from Swan Lake on the Illinois River and from Ellis Bay and Eagle’s Nest Island side channel on the Mississippi River.  Morphological characteristics of each individual were examined and genetic techniques (isozyme electrophoresis and Mitochondrial DNA sequencing) were used to determine the evolutionary origin of each individual.  The genetic techniques showed there was sufficient genetic variation to distinguish the two species, first generation hybrids and post-first generation hybrids.  Hybrids were collected from all three sites, with 27 of the 120 individuals classified as hybrids.  Female silver carp are more likely to hybridize.  The first generation of hybrids was reliably identified by twisted gill rakers but this characteristic was not reliable for subsequent generation.  Lamer et al. argue that it is possible hybridization is more common than observed during the study.  It is possible that in the study area, the mixing of genes between species could result in a carp population dominated by hybrids.  Post-first generation hybrids exhibit reduced fitness, reduced food conversion efficiency, lower disease resistance and reduced jumping behavior.  For these reasons, the authors argue that hybridization may ultimately reduce the effect of invasion.

Implications for anglers:  The high percentage of hybridization and the reduced fitness of hybrids have a potential to reduce the impact of an Asian carp invasion on Lake Michigan.  Decreased fitness of hybrids will reduce competition with native filter feeders.  The reduction in jumping behavior reduces the risks to anglers and boaters.  Also fisheries managers can use the insights into carp life history from this study to develop a plan to mitigate the impacts of a carp invasion, similar to the way fisheries managers used sterilized lampreys to reduce the lamprey population.        

Selected definitions:

Interbreed:  Breed different species or varieties of animals or plants; producing hybrids.

Hybrid:  An organism that is the offspring of breeding plants or animals of different species or varieties of animals or plants.

Fitness:  The contribution of an individual to the gene pool of the next generation, relative to the contribution of other individuals.

Taxonomic identity:  The classification of organisms according to their natural relationships.

Morphological characteristics:  Features of an organism’s form and structure.

Isozyme electrophoresis:  A process in which enzymes that catalyze the same chemical reaction but differ in amino acid sequence are separated by moving down an electrical field.

Mitochondrial DNA sequencing:  The process of determining the nucleotide sequences of DNA from mitochondria (a structure in cells that is the site of cellular respiration).

Gill rakers:  Bony or cartilaginous, finger-like projections off the gill arch, located in the gill cavity, which function in filter-feeders in retaining food.

Filter-feeders:  Organisms which feed by straining suspended matter and food particles from water.

Life history:  The series of events from birth through reproduction and death.

Sterile:  Unable to produce offspring.

Lamer, J.T., C.R. Dolan, J.L. Petersen, J.H. Chick and J.M. Epifanio.  2010.  Introgressive hybridization between big head carp and silver carp in the Mississippi and Illinois rivers.  North American journal of fisheries management.  30:1452-1461.