- University of Connecticut
- Department of Natural Resources and the Environment
- Christopher J. Sullivan
Apr 2014 - Dec 2016
- Graduate Research Assistant
- Broadly, I am interested in modes of range expansion of these species in the Mississippi River and the role large-scale climatic processes have on dynamic rates.
Research Items (7)
- Oct 2015
A variety of gears and analytical methods can be used to characterise lentic fish assemblages; however, the combined influence of gear type and analysis can affect conclusions about assemblage patterns. Fish assemblages sampled with night electric fishing, gillnets and trapnets from 153 lakes were evaluated using summary indices of species composition, pairwise community similarity comparisons and multivariate ordination. For a given amount of effort, electric fishing had the highest species richness, while gillnets had higher diversity and evenness. Pairwise comparisons between gears revealed that (1) richness was positively correlated among all gears, (2) diversity and evenness were generally not correlated across gears and (3) electric fishing and trapnets captured more similar species than all other pairwise comparisons. Gear-specific multivariate correlation and ordination revealed that gillnet and electric fishing samples more similarly characterised variation in assemblages among lakes, while trapnets characterised assemblages along different gradients of species composition. These results indicate that either electric fishing or trapnetting can be used when assessing shallow-water assemblages for diversity or evenness, gillnets and either electric fishing or trapnets should be used for whole-lake assessments, and either gillnets or electric fishing should be used when evaluating regionwide variation in lake assemblages.
- May 2017
Since their introduction in the 1970s, Silver Carp Hypophthalmichthys molitrix have spread throughout the Mississippi River basin. Management of any species relies on an accurate understanding of population characteristics and dynamics. However, Silver Carp seasonal sampling variation is unknown. Sampling during periods of peak catch rates would facilitate Silver Carp assessment and management, improving monitoring and removal techniques. The objective of this study was to evaluate adult Silver Carp seasonal sampling variation with boat electroshocking and trammel nets. Silver Carp were collected monthly (April–October) during 2014 and 2015 from four locations in the Des Moines River, Iowa, USA. Trammel nets rarely captured Silver Carp (mean = 4.9 fish/net ± 1.6 SE; 60% of fish captured in 6.3% of net sets) and therefore were not included in analyses. Electroshocking catch rates (CPUE) exhibited a bimodal distribution, with peak CPUE generally occurring in May, June, and September with lower catch rates during July and August. Catch rates were positively related to river discharge at upstream sites but not at downstream sites. Silver Carp size structure was similar among months and sites except at Cliffland where fish were smaller during August and October compared to earlier in the year. Finally, Silver Carp condition peaked during April and May and decreased throughout the year, except at Keokuk where peaks were observed during both May and August. Albeit substantial spatiotemporal variability, these results suggest that sampling Silver Carp with electroshocking in May–June and September-October results in generally higher catch rates compared to July and August and a more representative size structure. Using site-specific knowledge, monitoring and surveillance programs could more effectively sample during these periods of high vulnerability and densities in order to manage the spread and impacts of Silver Carp at state- and region-wide scales. Received 07 Dec 2016 accepted 11 May 2017 revised 24 Apr 2017
- Mar 2018
Recruitment of many fish populations is inherently highly variable inter-annually. However, this variability can be synchronous at broad geographic scales due to fish dispersal and climatic conditions. Herein, we investigated recruitment synchrony of Silver Carp Hypophthalmichthys molitrix across the Mississippi River basin. Year-class strength (YCS) and synchrony of nine populations (max linear distance = 806.4 km) was indexed using catch-curve residuals correlated between sites and related to local and regional climatic conditions. Overall, Silver Carp YCS was not synchronous among populations, suggesting local environmental factors are more important determinants of YCS than large-scale environmental factors. Variation in Silver Carp YCS was influenced by river base flow and discharge variability at each site, indicating that extended periods of static local discharge benefit YCS. Further, river discharge and air temperature were correlated and synchronized among sites, but only similarities in river discharge was correlated with Silver Carp population synchrony, indicating that similarities in discharge (i.e., major flood) among sites can positively synchronize Silver Carp YCS. The positive correlation between Silver Carp YCS and river discharge synchrony suggests that regional flood regimes are an important force determining the degree of population synchrony among Mississippi River Silver Carp populations.
- Nov 2018
Despite the longstanding presence of grass carp Ctenopharyngodon idella in the Upper Mississippi River (UMR) watershed, information regarding their populations remains largely unknown, in part because capture is difficult. Occupancy models are a popular wildlife assessment tool to account for imperfect detections but have been slow to be adopted in fisheries. Herein, we used occupancy modelling to evaluate the influence of two environmental covariates (river discharge and water temperature) on grass carp occupancy, extinction, colonization, and detection at nine sites within south‐eastern Iowa rivers from April to October 2014 and 2015. Grass carp were detected at least once at all but one site. The most parsimonious model indicated that grass carp colonization probability increased from 0.15 to 0.67 with increases in river discharge. In contrast, occupancy (0.20), extinction (0.29), and detection (0.50) probabilities were temporally constant. Models indicated that water temperatures did not influence grass carp extinction or colonization probabilities relative to river discharge. Cumulative grass carp detection probability approached 1.0, whereas conditional occupancy estimates were less than 0.1 when using five or more sampling transects. The use of a robust design occupancy model allowed us to estimate site occupancy rates of grass carp corrected for imperfect detections, while demonstrating the importance of river discharge for site colonization. These results can be used to assess the distribution of a cryptic fish while helping to guide grass carp sampling and removal efforts.
The introduction and spread of invasive species has been considered the most serious and least reversible threat to ecosystem function and native biodiversity. Since their introductions in the 1970s, Bighead Hypophthalmichthys nobilis and Silver Carp H. molitrix (collectively Asian Carp) have spread throughout the Mississippi River basin and become two of the most recognizable invasive species in North America. Their migration and possible establishment into reaches of the Upper Mississippi River (UMR) and its major tributaries is not well understood and knowledge of factors influencing population characteristics and dynamics is needed to facilitate assessment and management. Therefore, the objectives of this study were to evaluate 1) Asian Carp seasonal sampling variation (relative abundance, size structure, and condition) along the Des Moines River in southeastern Iowa and 2) the role of climatic variability in inducing synchronous fluctuations in Silver Carp recruitment in the Missouri, Des Moines, Mississippi, Illinois, and Wabash rivers. In chapter one of my thesis, Silver Carp populations were collected monthly (April – October) during 2014 and 2015 from four locations in the Des Moines River with boat electroshocking and trammel net sets. Trammel nets rarely captured Silver Carp (mean = 4.9 fish/net ± 1.6 SE; 60% of fish captured in 6.3% of net sets) and therefore were not included in analyses. Boat electroshocking catch rates (CPUE) exhibited a bimodal distribution with peak CPUEs in late-spring and mid-fall and lower in summer. River discharge was positively related to CPUE at upstream sites but the strength of the correlation decreased at downstream sites. Silver Carp size structure was similar among months and sites except at Cliffland where Silver Carp were smaller during the fall compared to earlier in the year. Finally, Silver Carp condition peaked during late spring and decreased throughout the year with the exception of Carp captured at Keokuk where bimodal peaks occurred during late-spring and early-fall. In chapter two of my thesis, Silver Carp were captured from nine sites across the Mississippi River watershed during June – September 2015. Silver Carp recruitment was asynchronous (mean ρ = -0.08 ± 0.13 SE) across the Mississippi River watershed. Populations <300 river km apart were synchronized but populations further apart or in relation to linear distance were asynchronous, suggesting dispersal may be occurring at local scales but local environmental conditions are more important determinants of recruitment at larger scales. Silver Carp recruitment was generally negatively related to variability in river discharge (6 of 9 populations) and positively related to cumulative river discharge during the reproductive season (7 of 9 populations), indicating that extended periods of elevated discharges are important local environmental factors regulating recruitment acting independently among sites. Regionally, Silver Carp recruitment was not significantly related to the North Atlantic Oscillation Index (NAO), El Niño Southern Oscillation Index (ENSO), or Palmer Hydrological Drought Index (PHDI), indicating that local environmental factors have more influence on Silver Carp year-class strength than broad climatic conditions. Collectively, these findings suggest that Silver Carp standardized sampling efforts should be concentrated during May-June or September-October when localized Silver Carp catch rates are highest and vulnerable to boat electroshocking. In addition, Silver Carp exhibit asynchronous fluctuations in recruitment and prevailing local environmental conditions regulate Carp year-class strength. As such, sampling efforts targeting Silver Carp populations during the spring may be beneficial as local abundances are highest and the removal of potential spawning stock biomass could aid in decreasing annual reproductive outputs, especially during years where environmental conditions are conducive to local reproductive success (i.e., sustained above average discharge rates). Collectively, understanding spatiotemporal fluctuations in Asian Carp dynamic rates will aid managers in determining how populations will respond to both local and regional environmental conditions, improving our understanding of a highly invasive and injurious fish species.
Invasive Silver Carp (Hypophthalmichthys molotrix) and Bighead Carp (H. nobilis; collectively called Asian Carp) are expanding throughout the Upper Mississippi River Basin (UMRB) and are of great concern due to their potential economic and ecological impacts. Pooled sections on the Upper Mississippi River associated with lock and dams may be poor habitats for reproduction and recruitment due to their lentic flow characteristics and perceived lack of adequate spawning habitat compared to more free-flowing unimpounded sections in the lower Mississippi River where evidence of reproduction has been documented. However, Iowa interior rivers connected to pooled sections of the Upper Mississippi River possess several requirements needed for successful spawning and observations of adults are becoming more prevalent. Unfortunately, little is known regarding the basic ecology and reproductive status of Asian Carp populations in these tributary systems. In order to properly make management decisions, information on reproductive success and factors that regulate populations must be understood. In this study, we evaluate 1) reproduction and recruitment patterns and 2) adult population characteristics and dynamics of Asian Carp in the Mississippi, Des Moines, Skunk, Iowa, and Cedar rivers in southeastern Iowa. Ichthyoplankton nets were used to capture 10,190 eggs and 38,868 age-0 fishes in 2014 and 5,929 eggs and 33,513 age-0 fishes in 2015 of unknown species from the Cedar, Iowa, Skunk, Des Moines and Mississippi rivers. In addition, 2,964 adult Asian Carp were collected from the Mississippi, Des Moines, Skunk, and Iowa rivers. Gonadosomatic index of female Silver Carp peaked in May 2014 and 2015 and were lowest in July during 2014 and August during 2015. Spawning females were not captured in 2014 but were captured from April to June in 2015. Post-spawn females were observed starting in June with 93% of females exhibiting spent gonads in July 2014 and were present in all months sampled in 2015. Egg densities peaked in late May and mid-June while age-0 fish densities were greatest during August in both years. Asian Carp eggs and age-0 fish were captured in the Skunk, Iowa, and Mississippi rivers in 2014. The highest densities of age-0 Asian Carp occurred within the tributaries and immediately downstream of each confluence with the Mississippi River. Adult Silver Carp were more numerous than Bighead Carp in all locations sampled. Catch rates of Silver Carp increased monthly during spring through fall, with low catch rates associated with high river flow in 2014 and 2015 as well as with low river flow in 2015. Asian Carp size and age structure varied among sites, with downstream sites having larger size and older age structures for Silver Carp and an opposing pattern for Bighead Carp. Additionally, Asian Carp growth rates varied among sites. Egg, larval, and adult samples will continue to be processed and additional data analysis will be conducted in 2016 to evaluate spatiotemporal patterns of reproduction and adult population characteristics.
Invasive Silver Carp (Hypophthalmichthys molotrix) and Bighead Carp (H. nobilis; collectively called Asian Carp) are expanding throughout the Upper Mississippi River Basin (UMRB) and are of great concern due to their potential economic and ecological impacts. Pooled sections on the Upper Mississippi River associated with lock and dams may be poor habitats for reproduction and recruitment due to their lentic flow characteristics and lack of adequate spawning habitat compared to more free-flowing unimpounded sections in the lower Mississippi River where evidence of reproduction has been documented. However, Iowa interior rivers connected to pooled sections of the Upper Mississippi River possess several requirements needed for successful spawning and observations of adults are becoming more prevalent. Unfortunately, little is known in regards to the basic ecology and reproductive status of Asian Carp populations in these tributary systems. In order to properly make management decisions, information on reproductive success and factors that regulate populations must be understood. In this study we will evaluate 1) reproduction and recruitment patterns and 2) adult population characteristics and dynamics of Asian Carp in the Des Moines, Iowa, Cedar, Skunk, and Mississippi rivers of southeastern Iowa. During 2014, ichthyoplankton nets were used to capture 10,190 eggs and 38,868 age-0 fishes of unknown species from the Cedar, Iowa, Skunk, Des Moines and Mississippi rivers. In addition, 1,152 adult Asian Carp were collected from the Mississippi, Des Moines, and Skunk rivers. Gonadosomatic index of female Silver Carp in the Skunk, Des Moines, and Mississippi rivers peaked in May and exhibited post-spawn gonads starting in June with 93% of females exhibiting spent gonads in July. Mean egg densities peaked in late May and mid-June while mean age-0 fish densities were greatest throughout August. Egg and age-0 fish densities tended to increase from upstream to downstream locations within each tributary and were higher in the Mississippi River compared to Iowa tributaries. Egg densities tended to be higher in the thalweg compared to channel border and backwater areas. Adult Silver Carp were more numerous than Bighead Carp in all locations sampled, exhibiting bimodal catch rate distributions. Total catches of Silver Carp increased monthly, with high catch rates associated with low river flow. Silver Carp size and age structure varied among sites, with downstream sites having larger size and older age structures. Additionally, length-at-age plots revealed great differences in Silver Carp growth among sites. To better predict the further expansion and success of Asian Carp in the UMRB, additional egg, larval, and adult sampling will be conducted in 2015 to evaluate factors influencing the spatiotemporal variation in abundance of these life stages.