Some aspects of the ecology of non-native Brook Charr (Salvelinus fontinalis) in headwater streams of Otago, New Zealand

Abstract

Humans have been altering their environment for thousands of years both intentionally and unintentionally. The alteration of freshwater ecosystems has been extensive through the transfer of species across natural barriers and outside their native range. Salmonids were introduced into the Southern Hemisphere in the mid-19th century during a worldwide frenzy of plant and animal introductions known as the Acclimatisation Movement.

Seeking to populate the streams and rivers with useful species for food or sport, brook charr (Salvelinus fontinalis) were introduced to New Zealand during this time along with a number of other salmonids. The introductions were haphazard and not well documented, especially as sustained, repeated introductions from hatchery stock continued well into the 20th century. This thesis examines the introduction of brook charr to New Zealand in a historical perspective, then looks at the current distribution of brook charr and some of their life history, their diet and energetics, and also competitive interactions with native galaxiids in a laboratory experiment.

The highland streams of Otago received brook charr introductions starting in the 1880s and sporadically through the 20th century. A fishery did not develop but many populations remain somewhat cryptically present in headwater streams, mostly upstream of brown trout (Salmo trutta). Some of these populations were confirmed and have a direct overlap with the native non-migratory galaxiid species complex which is threatened or endangered.

Brook charr displayed a suite of life history traits in these streams including small adult size, early or delayed maturation, and short life span. The life histories were reflective of a combination of factors at each site and were mostly shaped by temperature, growth, and interspecific competition. Growth was influenced by water temperature and light. Fish in warmer water had good growth rates and early maturation. Fish that were sympatric with brown trout had poorer growth but still had early maturation that appears to be a resource allocation strategy. Fish in the cold, high altitude water had slower growth but lived longer and had delayed maturation.

An examination of the energy available as invertebrate drift showed differences in the available energy available to fish and that was used to model growth and compare to observed growth. All sites were found to have sufficient invertebrate drift to support the populations. Diet was also examined and analyses indicate a significant overlap between charr and trout.

Competitive interactions were measured between brook charr and galaxiids in the laboratory. Galaxiids changed their behaviour in the presence of brook charr in a way that would have negative implications for a population and would make cohabitation of a stream very unlikely. Given these conclusions brook charr will persist in Otago for decades or longer and will remain a threat to native fish.