Reproduction

Background

Knowledge of the reproductive characteristics of a species is key to both assessing vulnerability and to proper management.  For example, age-at-maturity and fecundity are important life history parameters as they can greatly affect a species’ resilience to exploitation and extinction.  However, despite recent reported declines in abundance of skate in Atlantic Canada waters, much remains to be learned about the reproductive biology of even the most common species.  A recent study carried out at the Canadian Shark Research Lab at the Bedford Institute of Oceanography aimed to estimate size- and age-at-maturity and fecundity, and to discern seasonal patterns in reproductive output for the winter skate (Leucoraja ocellata), the little skate (Leucoraja erinacea), the thorny skate (Amblyraja radiata), and the smooth skate (Malacoraja senta) on the eastern Scotian Shelf (ESS) (NAFO Divisions 4VsW) off Nova Scotia, Canada.  Click on one of the following links to learn more about how each of the following characteristics is examined in skates:

Maturity

Fecundity

Reproductive cycle

Maturity

In order to carry out reproductive analyses, skates are collected aboard commercial fishing vessels and research vessels (RV).  Trawls are the most effective means of capturing skate, although the larger species (especially thorny and barndoor skate) are also captured on long-lines by fisheries targeting other species, such as halibut (Hippoglossus stenolepis).  Fish are frozen onboard for detailed analysis of their reproductive tracts back at the laboratory.

Once back at the lab, skates are thawed, total lengths and total weights are taken, and the sex of each specimen is noted.  Gross morphological analyses are then carried out in order to classify each specimen into a maturity or spawning stage.  Both external and internal reproductive characteristics are examined.  For a detailed description and photos of each of the reproductive structures examined in male and female skates, go to the Internal Anatomy section of this website.  

In males, the length and “state” (or rigidity) of the claspers are determined.  The length of the clasper is measured on the inside edge from the point of insertion to the tip of the clasper.  Clasper rigidity (or the degree of calcification) is assessed as either “uncalcified” when the organ is soft and easily bent; “plastic” when the organ is flexible but showing some structure and development of the tip (or glans); or “calcified” when the organ is completely stiff and resistant to deformation.  In calcified claspers, the glans structures also open readily upon flexion.  Males with long, calcified claspers are classified as mature.

Other indices of maturity in males that are recorded during dissection are:

	Testes weight (including weight of the epigonal organ due to its close association with the gonads)
	The development (or bulging) of the testicular lobes
	The diameter and shape of the epididymis and the vas deferens at their widest points
	The presence of sperm in the lower part of the ductus deferens
	The presence, total number, and state (soft, semi-soft, hard) of alar thorns

 

The clasper tip (or glans) of a matur emale thorny skate (Amblyraja radiata).  In mature males, the glans pops open readily when flexed.	The right alar thorn patch of a mature male little skate (Leucoraja erinacea)

In females, it is difficult to tell whether a female is mature from the exterior alone, unless she has egg cases within her genital tracts visible as a pair of prominent swellings anterior to the base of the tail on either side of the midline.  As such, an examination of the internal reproductive organs is necessary.  The following reproductive organ characteristics are examined:

	Uterus diameter
	Shell gland diameter
	Ovary weight
	Shell gland weight
	Total number and state of ovarian follicles
	Average diameter of the five largest ovarian follicles
	Presence/absence and state of egg capsules

When egg capsules are found, they are removed from the reproductive tract of the mother, and egg capsule weight and yolk weight are determined.  Females are classified as mature when they have egg capsules within their reproductive tracts, or when they have yolked ova and shell glands similar in size to those of spawning stage females.  

Size- and age-at-maturity are then determined for each species using maturity ogives.  Maturity ogives are plots of proportion mature against size or age.  The inflection points of logistic curves fit to maturity ogives can be used to accurately estimate size- and age-at-maturity (Fig. 1).  Click on the image below to see maturity ogives for male and female little skate on the eastern Scotian Shelf.  (Note: this is a high resolution image, so it will take somewhat longer to display). 

Overall, results of work carried out to date the the Bedford Institute of Oceanography indicate that winter, thorny and smooth skate are relatively slow-growing, late-maturing, long-lived fishes, and are therefore vulnerable to over-exploitation.  For a look at how old each of these species gets and to find out at what sizes and ages males and females of each species mature, go to the In Atlantic Canada section of this website, click on the species of interest, and go to the Life History section of each page.

Fecundity

Determining fecundity (or annual egg production) in NW Atlantic skates is extremely difficult because they are “serial spawners” that spawn multiple times each year, throughout the year.  Mature females have large yolked ova and many small white ova of all sizes in their ovaries during every month of the year, with smaller eggs maturing constantly to replace those that are encapsulated and oviposited (see Female Reproductive Tract).  This reproductive strategy makes it impossible to use counts of the number of mature ova in the ovaries of each species during the months of peak production to estimate the maximum number of eggs produced between then and the end of the spawning season, a method that has been used to estimate fecundity in ray species.  As such, fecundity in skates is determined indirectly.  The size of “immature” ova is determined for each species using ova diameter frequency distributions, and then the number of ova with diameters exceeding this size (called “mature ova”) in the ovaries is determined for each mature female.  This number is averaged across all mature females sampled to estimate a potential fecundity value for each species.

The following potential fecundity values were obtained for winter, little, thorny and smooth skate on the eastern Scotian Shelf off Nova Scotia, Canada:

Reproductive cycle

Seasonal patterns of mating and reproduction can be determined by examining reproductive organ characteristics by month of collection.  In male and female skates, characteristics that have been found to change over the course of a reproductive cycle include:

	Average follicle diameter
	Ovary weight
	Shell gland weight
	Number of vitellogenic (or yolked) follicles
	Testes weight
	Seminal vesicle diameter
	Epididymis width

Peaks in these characteristics can be indicative of periods of enhanced reproductive activity, especially if peaks are coincident between males and females of the same species.  Other, detailed studies have used steroid hormone concentrations and testes histology along with morphological measurements of the reproductive organs of males and females to determine the reproductive cycles of the winter and thorny skate in the Gulf of Maine.  Most studies on skate to date indicate that they either reproduce year round (i.e. thorny skate Amblyraja radiata in the Gulf of Maine), or have partially defined reproductive cycles with one or two peaks when a greater proportion of the population is reproductively active (i.e. winter skate Leucoraja ocellata in the Gulf of Maine).  On the eastern Scotian Shelf, it appears as if thorny and smooth skate (Malacoraja senta) are capable of reproduction year round, with thorny skate showing potential peaks in May and October.  Unfortunately, a lack of mature female winter and little skate (Leucoraja erinacea) on the ESS precluded a determination of their reproductive cycles in this area.

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