Northern Pike: Ecology, Conservation, and Management History

4. Sampling Northern Pike Populations

135 135 Chapter 4 Sampling Northern Pike Populations Traditional Sampling Tools Used in Minnesota Experimental gill netting and trap netting have been the two principal techniques for sampling fish populations during lake surveys in Minnesota since lake survey methods were standardized in a manual by John Moyle and Charles Burrows in 1954. Early on, gill netting (Figure 4.1) was recognized as an effective method for sampling northern pike, and the method has been relied on for comparing pike populations among lakes as well as for monitoring population trends within individual lakes (Moyle et al. 1950; Moyle 1950). The “experimental” gill nets used in Minnesota are nets designed to catch a broad range of fish sizes; they have five panels,and each panel has a different mesh size that selects for different sizes of pike. More recent research has described size selectivity of the different meshes and other factors that affect gill-net catchability of pike. The research has illustrated that gill-net catch rates can track population trends. However, gill netting during summer months, which is the standardized lake survey period, has the important drawback of killing fish. In contrast,short-term gill-net sets during the middle of the day in spring (while water temperatures are still cold) stress the fish much less and can be used for “catch-and-release” gill netting. Both methods of gill netting can be used to sample a variety of depths and habitat types. sampling northern pike populations s a m p l i n g n o r t h e r n p i k e p o p u l a t i o n s 136 Types of habitat sampled by trap nets have been more limited since they are usually fished in shallow areas along shorelines. Trap netting during the summer months has generally resulted in low catch rates for northern pike (see Guy and Willis 1991; McInerny and Cross 2005),and thus summer trap-net catches have not been a useful monitoring tool. But use of the same trap nets early in spring during ice-out and spawning (Figure 4.2) can be a very effective technique for catching large numbers of pike in a short time. Electrofishing is a technique for temporarily stunning fish in the water using electricity from a boat-mounted generator; an electric field is created between electrodes hanging from the boat. Electrofishing has caused moderate injury rates for pike (12.5%) (Holmes et al. 1990) and is most effective in shallow water. Low catch rates from both electrofishing (McInerny and Cross 2005) and from small beach seines during most of the open-water season have precluded their usefulness for monitoring pike populations, but neither gear has been evaluated during the spawning period. FIGURE 4.1. Sketch of an experimental gill net set along the bottom of a lake (excerpted from MNDNR 1993). s a m p l i n g n o r t h e r n p i k e p o p u l a t i o n s 137 Systematic angling is another tool not used for routine monitoring of northern pike populations in Minnesota,for good reason. Angling catch rates are much more variable than gill-net catch rates,and angling catch rates did not track changes in pike population numbers in a small lake where pike numbers were manipulated (Pierce and Tomcko 2003b). Angling catch rates are probably influenced more by changes in availability of prey fish (Lux and Smith 1960),changing weather patterns,and the fish learning to avoid angling gear (Beukema 1970). On the other hand,creel surveys that monitor recreational fishing have been valuable tools for tracking changes in recreational fisheries,and many good-quality FIGURE 4.2. Ice-out trap netting along a shallow lake shoreline. s a m p l i n g n o r t h e r n p i k e p o p u l a t i o n s 138 creel surveys date back into the early 1950s in Minnesota. Cook and Younk (1998) provided an in-depth historical analysis of creel surveys from Minnesota’s lakes and streams. For pike,they showed seasonal distribution of the recreational harvests, sizes and ages of harvested fish,and harvest and release rates by anglers. While creel surveys are valuable for monitoring recreational fisheries, they are not necessarily good tools for monitoring fish...