Illinois Natural History Survey - University of Illinois

Growth & Survival of Nearshore Fishes in Lake Michigan

Sara M. Creque and Sergiusz Czesny

alewifeLarvae.jpg To better manage the nearshore fish assemblage it is important to identify the primary factors that regulate fluctuations in fish populations both spatially and temporally. By identifying factors that affect growth and survival of early life-stages of fish, we can generate models to allow managers to predict inter-annual fluctuations in the adult populations.

Common nearshore fishes in Lake Michigan include yellow perch and alewife. Understanding the ecological constraints placed on yellow perch year-class strength is critical as managers try to predict if and when the Lake Michigan yellow perch population will rebound from the series of year-class failures dating back to 1990. Similarly, understanding alewife recruitment dynamics is important because these planktivores are the primary food source of stocked salmonids in Lake Michigan. An ability to predict alewife year-class strength will help managers to determine appropriate salmonid stocking levels, and may be useful to predict negative interactions between yellow perch and alewife. Extending our knowledge on other species such as bloaters, Cyprinids, and rainbow smelt will provide additional information on the prey base for adult sport fishes, and a more complete picture of competitive interactions within the nearshore fish assemblage.

To address the goals of this research, fish, zooplankton, benthic invertebrate, and temperature data are collected at four sample sites. Because year-class strength is frequently set long before fish recruit to the adult stock or fishable population, we focus our fish sampling on the larval and juvenile stages. Identifying the factors that determine success at early life stages should help to predict fluctuations in abundance of adult fish populations. Site selection was based on a set of criteria that included water depth (3-10m), substrate composition (soft to sandy sediments), distance from shore (<2 nautical miles), and geographical location (north or south) on the Illinois shoreline. There are two sample clusters: one in the north near Waukegan Harbor and the other in the south near Jackson Harbor. Within each cluster there are two sampling sites. Sampling these two clusters allows us to study spatial differences and patterns in the nearshore aquatic communities within southern Lake Michigan.

The patterns observed after nine years of study demonstrate that mechanisms influencing fish assemblages and recruitment may operate at localized scales (i.e. <100 km) as well as temporally. Qualitative differences in abiotic (i.e. temperature) and biotic conditions (i.e. zooplankton, benthic invertebrates) that could influence fish recruitment and success have been observed between our north and south sampling clusters. We have observed large differences in the temperature regime between the north and south sites, suggesting that water temperature may account for some of the variation in biota observed between clusters and years. For example, yellow perch spawn and hatch in late spring, and the rate of spring warming can greatly affect the time of emergence and success of larvae. Peak larval fish abundances were generally observed earlier in the south cluster, which has a faster rate of spring warming, compared to the north cluster. In the north cluster we observe species that prefer cooler water temperatures, such as sticklebacks, bloaters, and the amphipod Diporeia, more often. neustonRinse.jpg

Zooplankton abundance and composition may also affect growth and survival of nearshore larval fish and thus recruitment to the adult population. The amount of food and timing of food availability are critical determinants of first-year growth and survival of fish (Miller et al. 1988). While zooplankton abundance has been similar between north and south sites, species composition has differed. Prey availability for larval fish is a concern in southwestern Lake Michigan because nearshore zooplankton densities have declined from > 500/L during 1988 to < 20/L recently.

Benthic invertebrates are important to the function of the aquatic community because they act as a benthic-pelagic link for many fish species. Many young-of-the-year (YOY) fish such as yellow perch, spottail shiner, and trout-perch rely on benthic invertebrates as primary or secondary food sources. The most significant biotic differences observed between the north and south were benthic invertebrate densities and taxa diversity. Such differences in prey availability likely affect growth of YOY and juvenile fish between these two areas, and thus influence recruitment success. Our bottom trawl data indicate that different factors may drive recruitment success of different species, and the relative importance of these factors may also vary from year to year. Understanding the degree to which biotic and abiotic factors act together to affect growth and survival of nearshore fishes in Lake Michigan can provide useful predictive information to managers as they strive to regulate harvest of important sport and commercial fishes.

southGN.jpg It is important to continue to investigate how ecological conditions vary temporally and within smaller spatial scales of the nearshore zone, and the effects these differences may have on growth, survival, and species composition of the entire nearshore fish assemblage. 

To determine if and where a gradient in benthic invertebrate and YOY fish abundance and composition occurs along the Illinois shoreline, we began sampling four additional sites in 2006. This will provide additional insight into the importance of benthic invertebrates in the diets of YOY fishes, and help identify favorable areas that may promote faster growth and higher survival of nearshore fishes. Continued monitoring of larval and juvenile fishes along with abiotic and biotic variables is needed to 1) determine what mechanisms play a role in regulating fish recruitment in Illinois nearshore waters, 2) explore the extent of recruitment variability across years and increase our understanding of why these fluctuations occur, and 3) develop appropriate mechanistic models to predict year-class strength of nearshore fishes to aide managers in making decisions for harvest regulations.

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