Does Sperm Limitation Take Place in the American Lobster Fishery and, if so, why?

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Project Type: 
Research
Project Number: 
R/CFR-11
Inception Date: 
2008
Completion Date: 
2010

Participants:

Winsor Watson UNH - Department of Biological Sciences Principal Investigator
Kari Lavalli Boston University Collaborator
Michael Clancy Boston University Collaborator

Students Involved:

Audra Chaput UNH - Department of Biological Sciences
Jason Goldstein UNH - Department of Biological Sciences
Tracy Pugh UNH - Department of Biological Sciences
Tom Langley UNH - Department of Biological Sciences
Kirby Johnson UNH - Department of Biological Sciences
Casey Tobin University of New Hampshire
Haley White University of New Hampshire
Abstract: 

Despite being one of the most productive and lucrative fisheries in the North Atlantic, there is continued concern that North American lobster stocks are overfished and that the models used to predict fishery trends need to be recalibrated. Many of these models are based on a certain proportion of the sexually mature female lobsters contributing a large number of new recruits to the fishery each year. However, several recent findings suggest that the full reproductive potential of a portion of the sexually mature females is not being met, possibly due to skewed sex ratios and a paucity of large males. If this is so, then egg-recruit models could overestimate annual recruitment, which is ill-advised, at best. The overarching theme of this project is to determine if sperm limitation is becoming a factor in specific regions of the American lobster fishery and if so, what are some of the implications for lobster populations and the fishery that relies upon them. If sperm limitation is a factor, then we will determine if it is correlated with certain characteristics of the local populations, such as sex ratio and size differences between males and females. A final goal of this project will be to investigate one possible mechanism that might give rise to sperm limitation: the inability of small males to mate with significantly larger females.

The major goal of this project is to extend and expand our preliminary work to determine where sperm limitation exists in New England waters and to determine if the extent of the problem correlates with certain characteristics of the local populations, such as sex ratio and size differences between males and females. This task will be accomplished by characterizing the size frequency distribution of male and female lobsters in at least six different, widely separated, regions and then determining for each region the percentage of reproductive females that are carrying sperm as well as the percentage of berried females that are carrying eggs that have not been fertilized. In the laboratory we will also determine how long females will carry egg clutches that are only partially fertilized.

Although many factors may contribute to sperm limitation, the actual mechanisms limiting sperm supply remain unclear. A second goal of this project will be to investigate one possible mechanism that might give rise to sperm limitation: the inability of small males to mate with significantly larger females. In the laboratory, pre-molt females will be housed with males of different sizes to determine if small males are capable of mating with larger females.

This combination of field studies in sites ranging from Rhode Island to Maine, and investigations of lobster mating in the laboratory, will provide information that is vital to more fully understanding the factors that may limit the number of recruits that actually enter the lobster fishery each year. Once we have an adequate understanding of this important subject, this knowledge can be used to more effectively manage this economically and biologically influential marine resource.

Objectives: 
1. Quantify the sex ratios and size differences between sexually mature male and female lobsters from at least 4 regions of the fishery.
 
2. Measure the percent of non-berried and berried females in each size class that are carrying sperm in the spring and fall of each year.
 
3. Determine the percentage of berried females in each region carrying fertilized eggs, as well as determining the percentage of eggs within a clutch that are fertilized.
 
4. Determine if small males are capable of: 1) mating with larger females and; 2) providing sufficient sperm to fertilize and entire brood.
 
5. In the laboratory, determine if eggs that are not fertilized are carried for a shorter period of time than fertilized eggs.
Methodology: 

Sea-sampling will take place in six different regions of the lobster fishery. Each region will be characterized in terms of the male:female sex ratio and the size differences between reproductive males and females. In addition, sperm samples will be taken from all females to determine if they have successfully mated and, if they are carrying eggs, egg samples will be removed to determine if all the eggs in the clutch were fertilized. Finally, in the laboratory, studies were be conducted to determine if eggs that are not fertilized fall off the abdomens of females and if small males are capable of successfully mating with larger females.

Rationale: 

Despite being one of the most productive and lucrative fisheries in the North Atlantic, there is continued concern that North American lobster stocks are overfished and that the models used to predict fishery trends need to be recalibrated. Many of these models are based on a certain proportion of the sexually mature female lobsters contributing a large number of new recruits to the fishery each year. However, several recent findings suggest that the full reproductive potential of a portion of the sexually mature females is not being met, possibly due to fishery induced skewed sex ratios and a paucity of large males. If this is so, then egg-recruit models could overestimate annual recruitment, which is ill-advised. The overarching theme of this project is to determine if sperm limitation is becoming a factor in specific regions of the American lobster fishery and, if so, what are some of the implications for lobster populations and the fishery that relies upon them.

Accomplishments: 
2014
NHSG research indicates some lobsters become sexually active before they are capable of reproducing
Recent studies suggest that lobsters are either maturing at a smaller size due to ocean warming or harvest pressure, or immature female lobsters are mating before they can actually reproduce. N.H. Sea Grant-funded researchers conducted experiments to determine if immature female lobsters were carrying sperm. More than 31 percent of the immature females were carrying sperm, indicating they had mated. These data indicate that some female lobsters become sexually active before they are capable of reproducing. Results from this research will help resource managers to improve their understanding of lobster reproduction and help derive a more accurate estimate of reproductive potential for abundance estimates.
2013

N.H. researchers determine American lobster spermatophore quality and weight relative to size

Several studies suggest that the full reproductive potential of some sexually mature female American lobsters is not being met, which would lead to inaccuracies in the egg-recruit models that are used to estimate the size of the lobster population. The relationship between male lobster size and spermatophore composition was previously unknown, and the variation in the quality of ejaculate produced by male lobsters has been previously described, but never quantified. N.H. Sea Grant-funded researchers conducted studies to address these information gaps and determine if sperm limitation is occurring in the lobster fishery. The 2013 results indicate that spermatophore weight increases with male size, but there is no relationship between male size and spermatophore composition. However, when males mate with multiple females in succession, there is a reduction in the amount of sperm they provide to each female. The findings from this research will help resource managers to improve their population estimates for American lobsters in New England waters.

Immature N.H. female lobsters engage in mating despite no obvious population benefit
While conducting research to determine if sperm limitation is impacting the American lobster fishery in New England, researchers assumed only sexually mature female lobsters were mating. However, a recent study indicated that immature females might also mate. In 2013, N.H. Sea Grant-funded researchers sampled mature and immature female lobsters from N.H., collected sperm when present and examined their ovaries. The results indicated that some immature females mate, while some mature females do not. Although it is currently unclear how mating that does not lead to successful reproduction would benefit the species, the results from this study will provide a clearer understanding of lobster mating dynamics that could lead to improved population modeling.

American lobster size discrepancies and female-skewed sex ratios in N.H. populations might impact reproductive success
The American lobster fishery in the U.S. provides management measures intended to protect females that carry eggs, which can lead to increasingly female-skewed sex ratios, particularly at larger lobster sizes. This may affect reproductive output if females become sperm-limited due to insufficient number of males or males that are of the appropriate size. In 2013, N.H. Sea Grant-funded researchers conducted studies to determine if male lobsters are able to successfully mate with females that are larger than they are, or when females greatly outnumber males. Their research indicates that both size discrepancies and female-skewed sex ratios might negatively impact reproductive success, suggesting that management policies should take both sexes into consideration to conserve lobster reproductive potential.

Method developed by N.H. Sea Grant researchers to collect and quantify American lobster sperm
N.H. Sea Grant-funded researchers developed a method to collect spermatophores from male American lobsters and quantify the amount of sperm that is present in each spermatophore. This method will help resource managers to determine if sperm limitation is occurring in the lobster fishery and thus improve population estimates.

2011

 
Researchers Develop Fluorometric Method to Measure Total Lobster Egg DNA
Researchers have developed a fluorometric method to obtain measurements of total egg DNA to determine if American lobster eggs have been fertilized or not. This methodology will help to make a more accurate assessment of the proportion of female lobsters that will actually contribute new recruits to the fishery.
 
Researchers Develop Method for Staining Lobster Eggs to Determine Fertilization
Researchers have developed a method for staining lobster eggs in order to visualize the nuclei and determine if the eggs have been fertilized or not. Both fresh and fixed eggs can be examined with this method. This improved technique will allow lobstermen to collect and preserve eggs for researchers during the normal course of their operations, leading to the availability of a much larger sample of eggs than in previous years to more accurately estimate lobster mating success.
 
Researchers Develop Method to Determine Successful Lobster Mating
NHSG-funded researchers have developed a reliable, non-invasive method for determining if a female lobster has successfully mated. The method involves researchers inserting a needle into the seminal receptacle of female lobsters to determine the presence or absence of a sperm plug (presence of a plug indicates presence of sperm). If no plug is present, the contents of the receptacle are removed and viewed under a microscope to determine if sperm is present. Sometimes the plug has not hardened so the latter approach is necessary. This methodology will help determine if sperm limitation is impacting the American lobster fishery.
 

Economic and societal benefits

 
The lobster fishery is one of the most lucrative and important fisheries in New England. Our long-term goal is to provide data that will enable the industry and managers to work together to maintain a sustainable fishery. The data we have gathered to date has already shed light on the normal reproductive cycles of lobsters in the field. As a result of these data, we are beginning to question the “traditional” view of lobster reproduction and thus, the way we manage the fishery. Ultimately, when we have rigorously examined our data, we expect that it will have a strong influence on our estimates of the reproductive biomass in New England waters, and thus the way we manage the fishery.
 
Number of coastal and marine issue-based forecast capabilities developed and used for management
 
Our data will be used extensively by managers. Two of the scientists involved in this project are involved with managing lobsters in Massachusetts so it is likely that our data will be used in an effective manner.
 
In addition, during the course of this project we have developed two new techniques. The first is a non-invasive method for determining if a female lobster has successfully mated. We are in the process of submitting this method to the Journal for Visualizing Experiments. The second is a method for staining eggs to visualize nuclei, and thus determine if they have been fertilized or not.
 
Information used by managers to improve ecosystem-based management
 
A very important aspect of lobster biology is the influence of temperature on their growth and reproduction. The size at which females reach sexual maturity is strongly influenced by water temperature. This relationship is poorly understood for males. When males and females reach sexual maturity has a huge impact on fishing regulations, which are based, in part, on a minimum legal size and this is based on knowledge of when lobsters, in a given region, reach sexual maturity. Our data suggests that either lobsters are maturing at smaller sizes than expected, or immature lobsters are mating. Either situation needs will have a large impact on how the fishery is managed.

Publications

Available from the National Sea Grant Library (use NHU number to search) or NH Sea Grant

Journal Article

  • Johnson, K., J. Goldstein and W. Watson. Two methods for determining the fertility status of early-stage American lobster, Homarus americanus, eggs. Journal of Crustacean Biology 31(4):693-700, October 2011.
  • Pugh, T., J. Goldstein, K. Lavalli, M. Clancy and W. Watson (2013). At-sea determination of female American lobster ("Homarus americanus") mating activity: patterns vs. expectations. Fisheries Research 147: 327-337, October 2013.
  • Goldstein, J., T. Pugh, E. Dubofsky, K. Lavalli, M. Clancy and W. Watson (2014). A noninvasive method for in situ determination of mating success in female American lobsters ("Homarus americanus"). Journal of Visualized Experiments 84:e50498, February 2014.

Thesis/Dissertation

  • Goldstein, J. (2012). The impact of seasonal movements by ovigerous American lobsters ("Homarus americanus") on egg development and larval release. Doctoral Dissertation, University of New Hampshire.
  • Pugh, T. (2014). The potential for sperm limitation in American lobsters ("Homarus americanus") as indicated by female mating activity and male reproductive capacity. Doctoral dissertation, University of New Hampshire.