Fish Total Length Measurement Error from Recreational Anglers: Causes and Contribution to Noncompliance for the Mille Lacs Walleye Fishery

To estimate angler noncompliance with size limits due to measurement error by anglers, we used estimates of measurement error derived from (1) lengths of walleye Sander vitreus tagged and measured by biologists subsequently caught and voluntarily measured by anglers and (2) from dead walleyes of known length that anglers voluntarily measured during interviews at lake access sites. We also investigated the potential for postmortem changes in fish length. Monte Carlo simulations suggested that angler measurement error accounted for 41-100% of observed angler noncompliance with a 14-in to 16-in harvest slot limit for Mille Lacs lake, Minnesota. Simulations underestimated harvest (retention) rates for walleyes of lengths approaching the harvest slot boundaries compared with actual measures by anglers of the tagged fish they caught; this suggests greater attentiveness of anglers at measuring fish near the slot limits or angler intent to retain protected fish. When reporting total lengths of the tagged walleyes they caught, more anglers rounded measurements to 1-in or 1/2-in increments, whereas more anglers rounded to 1/4-in or 1/8-in increments when reporting lengths of tagged walleyes they retained or when measuring dead fish during interviews. Length measurement errors may be compounded by postmortem physical changes in harvested fish. Although postmortem changes will not influence angler retention of fish, temporal changes in. the physical condition of fish may influence whether a creel clerk or conservation officer observes a fish as noncompliant. Understanding the level and characteristics of measurement error is important for understanding the factors influencing angler noncompliance with size limits.