Changes in fish growth following a water clarity increase in a Great Lakes connecting waterway

ABSTRACT Objective Water clarity can have direct but complex effects on fish and fish populations, including influences on foraging success and growth. Water clarity has increased in much of the Laurentian Great Lakes, with invasive dreissenid mussels being a major contributing factor to that increase. Our objective was to determine if and when the growth of fish species with different feeding strategies have changed in the St. Clair–Detroit River system (SCDRS), the connecting waterway between Lake Huron (upstream) and Lake Erie (downstream), and whether changes in growth align with changes in water clarity. Methods We used historical water clarity data and fish length at age data collected from the SCDRS during fisheries surveys. Change-point analysis was used to determine whether there were significant shifts in water clarity and fish length at age and when those shifts occurred. Results Change-point analysis found a change point in mean annual Secchi depth within our survey period (1993–2023) after 2007, which aligned closely to change points in mean annual length at age for species feeding on the lower food web (juvenile Lake Sturgeon Acipenser fulvescens and Yellow Perch Perca flavescens). Growth of species feeding at higher trophic levels (Smallmouth Bass Micropterus dolomieu and Northern Pike Esox lucius) either did not change over the study period or did not have change points aligning with water clarity changes. Conclusions Growth of fish species that rely on the lower food web declined when water clarity increased in the SCDRS, which shortly followed a regime shift in Lake Huron from 2003 to 2005. This study highlights the cascading downstream effects of food web shifts in the Great Lakes and the importance of invasive species control and prevention; the study also provides context to which fish species are most likely to be affected by increases in water clarity.