This project will explore how water temperature affects blue crab growth and will develop a temperature-dependent molt-process model that predicts impacts of temperature change on the growth process. Scientists will determine the relationship between size at maturity and reproductive output over the female lifespan. Using their model, researchers will run computer simulations to show how crab growth, age and size at maturity, and lifetime reproduction might change under different temperature conditions. These simulations will lend insight into the potential impacts of temperature changes on blue crab populations and may help inform resource management and stock assessment.

Abstract

Blue crabs support valuable fisheries throughout the U.S. Atlantic and Gulf coasts; U.S. coastwide blue crab landings in 2021 totaled over 53,000 metric tons for a wholesale value of over $240 million. Blue crab populations and landings have decreased in many areas where commercial fisheries exist. In addition, decreases in size at maturity have been observed in some areas. As female blue crabs have a terminal molt to maturity, and single-clutch fecundity is tightly linked to body size, these decreases in size at maturity have the potential to lead to reductions in reproductive output,  highlighting the need to understand the factors regulating growth and maturity in blue crabs. The overarching goal of this project is to quantify the impacts of changing temperatures on blue crab growth rates, size at maturity, and lifetime reproductive output. Specific objectives, and a summary of methods, are below: 

Objective 1: To experimentally quantify the dependence between growth parameters and water temperature and develop a temperature-dependent molt-process model for blue crab growth in the northern Gulf of Mexico. We will rear juvenile blue crabs under a range of temperatures to quantify the temperature-dependence of intermolt period (IMP), growth per molt (GPM), and overall growth rate. These data will be used to develop a molt-process model for blue crab growth, which will include two submodels: one for IMP and one for GPM. The molt-process model developed here will provide a framework for predicting growth rates, phenology, age of entry into the fishery, and age/size at maturity under both current and future conditions.

Objective 2: To quantify the relationship between size at maturity and reproductive output over the female reproductive lifespan. We will collect female blue crabs of a range of body sizes during the 2–3 d following the terminal molt and mating and observing reproductive output for the duration of their life. For each brood produced, we will assess fecundity, fertilization rate, and hatching success. These methods will generate data on relationships between body size and both single-brood and lifetime fecundity as well as improved understanding of spawning frequency, fertilization rates, and hatching success in this species. 

Objective 3: To integrate results of objectives 1 and 2 to simulate growth rates, size/age-at-maturity, and lifetime reproductive output under a range of current and predicted future thermal regimes. Specific scenarios simulated will be determined following discussions with relevant stakeholders, though will likely include scenarios based on observed water temperatures and scenarios based on regional sea surface temperatures under a range of climate change projections. 

Recent decreases in size at maturity have been observed in several areas, causing concern among crabbers, scientists, and managers. This study will provide insight into the mechanisms driving these decreases and the reproductive consequences. Additionally, the molt-process model and simulations  will provide information on predicted future changes in blue crab growth, phenology, size at maturity, and reproductive output under a range of climate change scenarios. Results of this study can be used to develop adaptive management plans for the fishery to ensure its continued sustainability.

Objectives

The overarching goal of this project is to quantify the impacts of changing temperatures on blue crab growth rates, size at maturity, and lifetime reproductive output. Specific objectives are as follows:

1. To experimentally quantify the dependence between growth parameters and water temperature and develop a temperature-dependent molt-process model for blue crab growth in the northern Gulf of Mexico. Measurable outcomes: (a) quantitative relationships between IMP, GPM, and water temperature, (b) determination of the number of molts from metamorphosis to maturity and the temperature-dependence of this parameter, and (c) a temperature-dependent molt-process model framework for predicting impacts of temperature change on the growth process.
2. To quantify the relationship between size at maturity and reproductive output over the female reproductive lifespan. Measurable outcome: a quantitative relationship between size at maturity and lifetime reproductive output.
3. To integrate results of objectives 1 and 2 to simulate growth rates, size/age-at-maturity, and lifetime reproductive output under a range of current and predicted future thermal regimes. Measurable outcome: simulated growth rates, size and age at maturity, and lifetime reproductive output under a range of thermal regimes. 

Methodology

Objective 1: We will experimentally assess the relationship between growth parameters (interment period, growth per molt, and overall growth rate) and water temperature throughout the juvenile period, and use these data to develop a temperature-dependent molt process model that can be used to predict growth rates, phenology, age of entry into the fishery, and age/size at maturity under both current and future conditions.

Objective 2: We will experimentally examine the relationship between size at maturity and lifetime reproductive output by collecting female blue crabs of a range of body sizes during the 2–3 d following the terminal molt and mating and observing reproductive output for the duration of their life. For each brood produced, we will assess fecundity, fertilization rate, and hatching success. These methods will generate data on relationships between body size and both single-brood and lifetime fecundity as well as improved understanding of spawning frequency, fertilization rates, and hatching success in this species. 

Objective 3: The temperature-dependent molt-process model (Obj. 1) will be used to simulate growth of juvenile blue crabs from metamorphosis to maturity, while the results of Obj. 2 will be used to simulate reproductive output once crabs reach maturity. Both current and future scenarios will be used, and each will include seasonal temperature variation. Specific scenarios simulated will be determined following discussions with relevant stakeholders, though will likely include scenarios based on observed water temperatures in Mississippi Sound over the past 15–20 years (e.g., average scenario, warm-years, and cold-years), and scenarios based on regional sea surface temperatures under a range of climate change projections. Simulations will provide insight into the potential impacts of interannual temperature variability as well as long-term climate change on blue crab growth rates, size and age at maturity, age of entry into the fishery, spawning phenology, and lifetime reproductive output. 

Rationale

Blue crabs, Callinectes sapidus, support valuable fisheries throughout the U.S. Atlantic and Gulf coasts; U.S. coastwide blue crab landings in 2021 totaled over 53,000 metric tons for a wholesale value of over $240 million (NMFS 2022). The blue crab fishery is consistently in the top 10 most valuable fisheries in the U.S., with the 8th highest landings by value in 2019 (NMFS 2020). Blue crab populations and landings have decreased in many areas where commercial fisheries exist (Lipcius and Stockhausen 2002, Eggleston et al. 2004, GSMFC 2013). In addition, decreases in size at maturity have been observed in some areas (Lipcius and Stockhausen 2002, Eggleston et al. 2004, Olsen et al. 2016). As female blue crabs have a terminal molt to maturity (Milliken and Williams 1984), and single-clutch fecundity is tightly linked to body size (Hines 1982), these decreases in size at maturity have the potential to lead to reductions in reproductive output,  highlighting the need to understand the factors regulating growth and maturity in blue crabs. 

Both field studies (Fisher 1999, Eggleston et al. 2004, Darnell et al. 2009, Hines et al. 2010) and laboratory experiments (Cadman and Weinstein 1988, Brylawski and Miller 2006, Cunningham and Darnell 2015) indicate that blue crab growth is highly sensitive to environmental conditions, particularly temperature. Recent evidence from studies on early juvenile blue crabs and related species (e.g., Cunningham and Darnell 2015, Kuhn and Darnell 2019) suggests that increased temperatures, as are likely to be seen in the coming decades, result in increased growth rates but decreased size at each juvenile stage. If these patterns continue through maturity, warmer water temperatures will reduce size at maturity, decrease age of entry into the fishery, and potentially reduce reproductive output of female crabs.