Eric G. Johnson

Release Strategies for Estuarine Species with Complex Migratory Life Cycles: Stock Enhancement of Chesapeake Blue Crabs (Callinectes sapidus)

Responsible stock enhancement requires rigorous experiments to develop release strategies that account for movement of all life-history stages among habitats across inshore-offshore and estuarine gradients. However, crab stock enhancement research to date has focused primarily on hatchery production, with only limited field assessments of the efficacy of releases to increase the target population. This paper summarizes ongoing research to develop effective release strategies for hatchery-reared juveniles to augment the spawning biomass of Chesapeake Bay blue crabs, which has declined > 80% in 15 years and appears to be recruitment limited. Our release experiments focused on three factors: (1) components of preparation and release, which included life stage and size at release, pre-release conditioning to minimize differences between hatchery and wild crabs, and micro-habitat and micro-timing of release; (2) stocking variables, particularly seasonal timing of release and stocking density; and (3) site selection and coordination, including release macro-habitat and location of release sites along environmental gradients, emphasizing coordination of release site and fishing pressure with migration corridors linking nurseries to spawning areas. In the first 5 years of research, we demonstrated that small (1,000–10,000) cohorts of hatchery reared, 20 mm, 7th-instar juvenile blue crabs can be tagged and released into small (1–10 ha) coves, and that these cohorts can be followed successfully to quantify growth, survivorship, and productivity of the enhanced population. We also determined the timing and routes of migration using a tag-reward system with the cooperation of fishers. Our multifaceted research strategy provides a model for responsible approaches to stock enhancement of other species with complex migratory life cycles.

Importance of season and size of release to stocking success for the blue crab in Chesapeake Bay

A critical step toward optimizing the success of stock enhancement is to identify release strategies that maximize survival and enhance growth of released hatchery-reared individuals. A key, but often overlooked, consideration for optimizing release strategies is the interaction of factors (e.g., release season, release size, stocking density, release habitat, etc.). Here, we summarize seasonal and size-dependent patterns of survival and growth for juvenile blue crabs using long-term field tethering and experimental releases of hatchery-reared cohorts within the Rhode and South Rivers, Maryland, USA, and review the direct and interactive effects of these factors. Survival of both tethered and free-ranging hatchery-reared juvenile crabs was high in early spring and fall and lowest in summer. Survival was largely independent of size during spring and fall, but increased with size in summer, indicating that optimal size at release varies seasonally. Hatchery-reared juveniles from spring releases grew rapidly, matured during their first season, and migrated to the spawning sanctuary in the fall of their first year. While release season and size each had direct effects on enhancement success, the results also highlight the important interaction between release season and size on enhancement success using the blue crab in Chesapeake Bay as a model.

Do Hatchery-Reared Blue Crabs Differ from Wild Crabs, and Does it Matter?

Successful use of hatchery-reared juveniles to enhance recruitment-limited populations or severely depleted stocks is contingent upon their ability to survive and grow upon release into the wild. Hatchery conditions often result in juveniles that exhibit morphological, physiological, or behavioral characteristics different from their wild counterparts. Managers of stock enhancement or restocking programs need to assess whether any such deficiencies translate into differences in performance between hatchery-reared and wild juveniles in the field. If so, the deficits may be minimized through conditioning strategies. In this review, we summarize various aspects of the morphology and behavior of cultured and wild blue crabs, Callinectes sapidus, and compare our work on these crabs to studies on other invertebrates. We identify similarities and differences between cultured blue crabs and wild conspecifics. In some cases where differences existed, conditioning of the hatchery-reared crabs before release rapidly mitigated the defects, and, overall, any remaining differences did not translate into decreased survival. We conclude that there are no significant impediments to the fitness of hatchery-reared blue crabs used in release programs.

Importance of blue crab life history for stock enhancement and spatial management of the fishery in Chesapeake Bay

Due to over-harvesting and habitat degradation, spawning stock abundance and biomass of the Chesapeake Bay blue crab (Callinectes sapidus) has declined over 80% in the last 15 years. In addition, only a small portion (11–22%) of the spawning stock migrates successfully to the historic spawning areas of the lower estuary. As a result, recent management goals to decrease harvests and increase spawning stock biomass have been adopted by the different Chesapeake Bay regulatory agencies. The crisis has also prompted an experimental assessment of the potential for stock enhancement to increase the number of spawners. While much of the current stock enhancement work has focused on assessing the competency of hatchery-reared individuals and identifying key processes that optimize survival and growth of juveniles to maturity, less attention has been paid to examining factors that influence the long-term efficacy of stock enhancement efforts. Here we discuss important interactions between blue crab life history, stock enhancement efforts, and management options available to increase the standing stock of mature females in Chesapeake Bay. We propose the establishment of migration corridors to protect female blue crabs when they undergo the long-distance migration after mating to lower Bay spawning areas.

Pilot Evaluation of Early Juvenile Blue Crab Stock Enhancement Using a Replicated BACI Design

We quantified whether local populations of early juvenile blue crabs (J1–2) could be enhanced through the translocation of crabs to underutilized nursery habitats, and if enhancement success, survival, and potential impacts of stocked crabs on their benthic prey varied in a density-dependent manner. Using plankton nets, ∼143,000 blue crab megalopae were collected as they ingressed into Pamlico Sound, NC. Of these, ∼13,800 early juvenile blue crabs (J1–2 stages) were then stocked at potential nursery sites relatively far removed (32–70 km) from their initial settlement areas using a replicated before-after control impact (BACI) experimental design. On average, there was negative enhancement success (−34%) five weeks after local crab enhancement, and no evidence of density-dependent enhancement success, mortality, or impact on potential crab prey. Poor stocking success was likely due to pelagic emigration from enhancement sites relative to controls. Attempts to assess the feasibility of stocking blue crabs at local scales of small coves should (i) probably not consider J1–2 stages because of their apparent propensity to emigrate from these areas, or (ii) further assess the effects of geomorphology and wind fetch of release sites on density-dependent emigration.

Age, growth and population structure of invasive lionfish (Pterois volitans/miles) in northeast Florida using a length-based, age-structured population model

The effective management of invasive species requires detailed understanding of the invader’s life history. This information is essential for modeling population growth and predicting rates of expansion, quantifying ecological impacts and assessing the efficacy of removal and control strategies. Indo-Pacific lionfish (Pterois volitans/miles) have rapidly invaded the western Atlantic, Gulf of Mexico and Caribbean Sea with documented negative impacts on native ecosystems. To better understand the life history of this species, we developed and validated a length-based, age-structured model to investigate age, growth and population structure in northeast Florida. The main findings of this study were: (1) lionfish exhibited rapid growth with seasonal variation in growth rates; (2) distinct cohorts were clearly identifiable in the length-frequency data, suggesting that lionfish are recruiting during a relatively short period in summer; and (3) the majority of lionfish were less than two years old with no lionfish older than three years of age, which may be the result of culling efforts as well as ontogenetic habitat shifts to deeper water.