Introduction 197Methods 197 Model formulation 198Parameter value specification 199 Results 200Discussion 203Abstract Fishery collapses cause substantial economic and ecological harm, but common management actions often fail to prevent overfishing. Minimum length limits are perhaps the most common fishing regulation used in both commercial and recreational fisheries, but their conservation benefits can be influenced by discard mortality of fish caught and released below the legal length. We constructed a computer model to evaluate how discard mortality could influence the conservation utility of minimum length regulations. We evaluated policy performance across two disparate fish life-history types: short-lived high-productivity (SLHP) and long-lived low-productivity (LLLP) species. For the life-history types, fishing mortality rates and minimum length limits that we examined, length limits alone generally failed to achieve sustainability when discard mortality rate exceeded about 0.2 for SLHP species and 0.05 for LLLP species. At these levels of discard mortality, reductions in overall fishing mortality (e.g. lower fishing effort) were required to prevent recruitment overfishing if fishing mortality was high. Similarly, relatively low discard mortality rates (>0.05) rendered maximum yield unobtainable and caused a substantial shift in the shape of the yield response surfaces. An analysis of fishery efficiency showed that length limits caused the simulated fisheries to be much less efficient, potentially exposing the target species and ecosystem to increased negative effects of the fishing process. Our findings suggest that for overexploited fisheries with moderate-to-high discard mortality rates, reductions in fishing mortality will be required to meet management goals. Resource managers should carefully consider impacts of cryptic mortality sources (e.g. discard mortality) on fishery sustainability, especially in recreational fisheries where release rates are high and effort is increasing in many areas of the world.

We evaluated the effects of dam removal on fish assemblage structure and spatial distributions after four low-head dam removals in the Baraboo River, Wisconsin, using data collected at 35 study sites over 7 years. After dam removal, biotic integrity scores (possible range ¼ 0-100) increased by 35-50 points at three of the four former impoundments as a result of decreases in percent tolerant species, increases in the number of intolerant species, and in some cases, increases in species richness. Fish assemblage shifts were muted at a fourth, lower-gradient impoundment site, indicating that responses differ among dam sites within a river system. In tailwater areas, postremoval assemblage shifts were transient; biotic integrity and species richness declined initially but then recovered at two of the three sites within 2 years after dam removal. An analysis of spatial distributions before dam removal revealed 11 fish species that were found below, but rarely or never above, the downstream-most dam. After dam removal, 10 of the 11 species were collected at new sites upstream from the dam, indicating that recolonization of reconnected upstream sites had occurred. Some species recolonized rapidly and in large numbers. For example, emerald shiners Notropis atherinoides recolonized 16 upstream sites and were collected 123 km upstream from the dam within the first year after removal. One of the nine recolonizing species, the spotted sucker Minytrema melanops, was only detected during spring, suggesting that these fish recolonized seasonally, perhaps for spawning. Recolonizing species were generally lacustrine or large-river fishes known to undergo overwintering and spawning migrations. Our study suggests that dam removal is a viable option for restoring lotic fish communities, but further study is needed on recovery patterns as they relate to channel morphology, hydrologic characteristics, impoundment sediment storage capacity, and the distance to source populations of recolonizing taxa.

Prospectos de un colapso inducido por pesca en la carpa asiática del Río Illinois RESUMEN: . La carpa asiática amenaza con invadir el Lago Michigan a través del sistema de vías acuáticas del área de Chicago, lo cual podría acarrear serias consecuencias en las tramas tróficas de los grandes lagos. Además de los esfuerzos llevados a cabo para impedir el ingreso de estos peces al Lago Michigan mediante barreras eléctricas, el estado de Illinois ha iniciado un programa de pesca en el Río Illinois, cuya finalidad es reducir la densidad poblacional a través de una pesca comercial intensiva. En este estudio se exploran los prospectos de un colapso de la carpa asiática por medio de un régimen de pesca intensiva. Sobre la base de un meta-análisis de datos demográfi-cos, se desarrolla un modelo de simulación dinámica para comparar el desempeño de estrategias de explotación tanto reales como alternativas para el Río Illinois. Las proyecciones del modelo sugieren que, de mantenerse las tasas de captura por debajo de 0.7 o si la pesca continua siendo selectiva a tallas (dirigiéndose a peces >500 mm o <500 mm) o a especies (carpa cabezona), entonces es poco probable colapsar la carpa asiática en el Río Illinois, aunque la biomasa se puede reducir considerablemente. Se argumenta que, pese a lo anterior, es posible lograr el nivel de esfuerzo pesquero predicho por el modelo, necesario para colapsar las poblaciones de la carpa asiática, si la pesca comercial se expande y se combina con incentivos económicos con tal de mejorar la selectividad a la talla y a las especies objetivo. ABSTRACT: Invasive Asian Carp are threatening to enter Lake Michigan through the Chicago Area Waterway System, with potentially serious consequences for Great Lakes food webs. Alongside efforts to keep these fishes from entering LakeMichigan with electric barriers, the state of Illinois initiated a fishing program aimed at reducing their densities through intensive commercial exploitation on the Illinois River. In this study, we explore prospects for the "collapse" of Asian Carp in the Illinois River through intensive fishing. Based on a meta-analysis of demographic data, we developed a dynamic simulation model to compare the performance of existing and alternative removal strategies for the Illinois River. Our model projections suggest that Asian Carp in the Illinois River are unlikely to collapse if existing harvest rates are kept below 0.7 or fishing continues to be size selective (targeting only fish >500 mm or <500 mm) or species selective (targeting mostly Bighead Carp), although their biomasses could be greatly reduced. We argue that it would still be possible to achieve fishing effort targets predicted by our model to collapse the Asian Carp populations if efforts to expand commercial fishing are combined with economic incentives to improve size selectivity and species targeting.