Lake Michigan yellow perch have had poor recruitment since the late 1980’s. The year classes of 1998 and 2002 were marginal, at best, fair. If the recruitment trends continue in this manner, the yellow perch populations in Lake Michigan could become critically low. This has biological implications within the entire lake food web. Ascertaining the cause for this is the objective of the Yellow Perch Task Group and the purpose of this research. We propose that a possible explanation for this population decline involves zebra mussel-induced habitat change and transport of pelagic phase yellow perch away from preferred habitat to poor habitat by strong meso-oceanic currents.

Since the introduction of zebra mussels into Lake Michigan in the early 1990s, the habitats for yellow perch have changed dramatically. Zebra mussels have become well established in rocky areas, predominantly on the west side of the lake, and have enhanced food resources in these areas by increasing productivity in the benthos. In areas with soft bottoms, predominantly on the east side of Lake Michigan, zebra mussels have depleted fish food resources, particularly the benthic amphipod Diporeia, by filter feeding phytoplankton upon which the invertebrates had fed. This change in food web structure may be a major impetus for the decline in the young yellow perch population. For more information on the effects on fish food resources, please reference studies by Brandt, Mason et al, and Vanderploeg on the NOAA-GLERL web site http://www.glerl.noaa.gov/

The alteration in habitat value to yellow perch indicates that rocky habitats are improved while soft habitats are worsened. This does not in itself explain all the interactions leading to the decline of yellow perch. However, because Lake Michigan more closely resembles an inland sea, rather than a lake, the winds generate strong currents and waves. Yellow perch evolved in much smaller bodies of water and that evolutionary process produced the current life history in which the tiniest yellow perch, newly hatched, spend about 40 days or more adrift in the water column. High wind or current activity, such as is common on Lake Michigan, disperse these larval fish great distances offshore to potentially unproductive habitats. Evidence documenting the extent of transport of young yellow perch has been accumulating, and the most recent material is presented through this research. Although this is a relatively unknown situation in freshwater, it is a common problem faced by many marine fishes, including those from coral reefs. The pelagic phase larval fish are carried away from their optimal, or preferred, habitats by currents or warm-core rings (Hare et al 2002, Limnology and Oceanography 47(6): 1774-1789). Some tropical and subtropical juvenile species end up marooned in the Gulf of Maine!

Figure 1: Larval perch transport following hatching. Numbers along the x-axis indicate distance east or west from the Milwaukee River mouth (0km) longitude. The y-axis is total length (mm) for each individual. In each box plot, the box is quartiles, the lines are 75%, and the dots are outliers. Notice the increase in distance with time for an 11-day period.

In 2002 there was good documentation of transport of pelagic perch. These collections have been opportunistic, piggybacking with other cruises. This information is summarized in the adjacent graph (Fig 1). The larval perch were collected by towing for 15 minutes with a large net (2 m wide and 1 m deep) at the surface of the lake at night on the dates indicated. The fish in the first graph (June 18, 2002) have been adrift for approximately 3 days and still retain the majority of their yolk sacs. About a week later (June 24, 2002) the young perch have drifted and are most abundant about 16 km (10 miles) offshore. This is about 1/8 of the way to the Michigan shore. These larvae were inferred to have originated in a rocky area due to the large amount of Cladophora algae, which is shed from rocks, associated with them. They may have been farther off shore, but no more sampling was possible due to cruise restrictions. A few days later (June 27-29, 2002) the young fish were found in good numbers as far as 41 km (about 25 miles) offshore, or about 1/3 of the way to the Michigan shore. Note that the larger, and probably older, individuals are farther offshore, having been adrift and feeding longer. These data indicate that during early summer when strong winds and currents are not uncommon, it is feasible that pelagic phase yellow perch can disperse widely from their hatchery, perhaps even across the lake.

What are, if any, the limitations to drift of these pelagic phase yellow perch? Can the young perch navigate back to viable habitats from offshore? These questions remain unanswered and require further investigation.

Evidence of the importance of rocky habitat for yellow perch has also been accumulating. Adults prefer rocky habitat (Wells 1977 J. Fish. Res. Board Can. 34: 1821-1829) perhaps due to greater food sources as their stomachs are more filled in these habitats (Wells 1980 Technical paper). Yellow perch eggs are more abundant in rocky areas (Robillard and Marsden 2001). Whether this is because the adults are already primarily on rocky habitat or they actively seek rocks for spawning is not known. On the west side of Lake Michigan, from the Indiana-Illinois border north to the Wisconsin border and also from Racine, WI, to north of Milwaukee, the bottom is rocky, consisting of bedrock fossil coral reefs and rubble transported by the glaciers long ago. This provides good feeding and spawning habitat.

Map 1: Map of perch sampling in Lake Michigan. Cross-hatched areas are rocky habitat, suitable for spawning and preferred area for yellow perch feeding. Red dots indicate locations for larval perch collection (Figs 1-3). Blue dots indicate shore sampling for young perch in rocky and sandy habitats (Fig 4).

In 2002 we documented the first evidence that post-pelagic perch—those seeking suitable bottom habitat as feeding grounds and shelter—are also preferentially found on rocks. The preferred habitat, as for adults, is rocky habitat. We documented a preference for rocky habitat in newly demersal YOY yellow perch by setting micromesh gill nets on four different sites with rocky versus sandy habitats (map 1). The relative catch rate for numbers of juvenile yellow perch captured in gill nets at rocky sites was approximately four times greater than at sandy sites in Wisconsin during mid-August through mid-September (Fig 4).

Yellow perch are schooling fish, which causes great variation in catch from day to day. Despite this, the trend is still apparent.

 

Figure 4: Percentage distribution of young yellow perch on rocky versus sandy habitats. Rock, shown as the darker bars, is preferred over 4 times than sand, the lighter bars. The actual numbers found at each location are above each corresponding bar.

Diet analysis was conducted on the young fish captured from the rock and sand habitats. The diet analysis is ongoing, but the preliminary analysis shows that the young perch on the rocks primarily consumed amphipods (33%) and isopods (43%), prey associated with rocks. These rocks are host to abundant yellow perch food, including small crustaceans and aquatic insects, crayfish, and sculpins. Those fish found on sand most likely were traveling between rocky areas. This is also evidenced by the presence of amphipods in their diets, which are associated only with rocks. Unfortunately, the rocky areas remain relatively unstudied. Many questions still remain unanswered regarding rocky habitats and the extent of pelagic phase fish transport by currents and waves. More study opportunities are needed; as information is accumulated, it will be included at this site.