The impact of vessel traffic on riverine fish

Passenger ship of Köln-Düsseldorfer on the river Rhine. Credit, Frila. Findings from a new study suggest that the currents created by vessels navigating river channels can have negative impacts on certain fish.

Researchers looked at a large river system in lowland Germany and found that impacts on young-of-the-year (YOY) fish varied depending partially on their life history characteristics. Roach whose larvae nurse in the benthic layer (i.e at the bottom) near the shoreline were more heavily impacted than perch whose larval stage is in the pelagic zone (i.e. open water).

As navigation increases in these channels, mitigation measures may be required to reduce population level impacts on fish assemblages. Fish can be negatively impacted by vessel traffic in any number of ways including: direct contact leading to mortality, pollutants and toxins from fuels and paints, and increased sediments in the water column.

In channels of limited width, currents from large commercial vessels may cause rapid and localized changes in current patterns. This may lead to changes in fitness of YOY fish through displacement into areas of lower quality or dewatering that may expose larvae to air. Fish using these nearshore areas as nursery habitats can experience lower recruitment due to this vessel traffic.

Researchers conducted electrofishing sampling at numerous sites biweekly between May and September. Zooplankton was also collected along with water temperature and oxygen content and a number of additional abiotic variables. Passage records for the channels were used to calculate navigation intensity. Multivariate tests including hurdle regressions, were used to test a number of abiotic factors predicting occurrence and distribution of fish assemblages.

Results of the analysis revealed that these two fish responded differently to increasing navigation intensity - roach exhibited a decrease in density while perch density remained constant. This collaborates a number of previous studies that found perch dominate fish assemblages in waterways where both occur in areas of high vessel traffic pressure.

Due to the correlative nature of this study the mechanism for this differential survival is not certain. Analysis strongly points to navigation pressure being the likely candidate. Zooplankton sampling seems to rule out effects of turbidity as it was found to have higher abundance in areas of high traffic.

However, potential toxic compounds in the water were not analyzed and may tell part of the story. Ship noise may also impact the fish differently. Regression models seem to rule out the effects of recreational vessels. 

Overall, the study suggests that commercial vessel traffic is negatively affecting native fish populations associated with channel bank habitats. As this pressure is predicted to increase in coming years, investigation in to potential mitigation measures is definitely merited. 

--by Jason Thompson

Huckstorf, V., Lewin, W., Mehner, T., & Wolter, C. (2010). Impoverishment of YOY-fish assemblages by intense commercial navigation in a large Lowland river River Research and Applications DOI: 10.1002/rra.1420