Winter skate

Winter skate
Conservation status
Endangered  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Rajiformes
Family:	Rajidae
Genus:	Leucoraja
Species:	L. ocellata
Binomial name
Leucoraja ocellata Mitchill, 1815

The winter skate (Leucoraja ocellata) is a globally endangered species of skate found in the surrounding waters of northeastern North America. They inhabit shallow shelf waters and are prone to bycatch during commercial fishing. There are three populations or Designable Units (DU) including the Gulf of St. Lawrence population (GSL), the Eastern Scotian Shelf and Newfoundland population (ESSN), and the Western Scotian Shelf population.

Description

The winter skate can reach a maximum size of 109 cm. It reaches maturity around 12 years of age, depending on the sex and area.

The skate's fins are used for swimming, while its tail remains stiff during most movements.

Winter skates are light brown and have small dark spots along the dorsal side of their body. L. ocellata have small spines covering most of their disk and tails.

L. ocellata usually have one to four ocelli, eye-like markings, on the upper surface of the disk, which each have a dark brown center and are pale around the edge. Their upper jaw has 72 rows of teeth

In males, 50% maturity occurs at a total length of 730 mm at 11 years old. For females, 50% maturity occurs at a total length of 760 mm between 11 and 12 years of age. It is a relatively late-maturing, long-lived species, making it vulnerable to exploitation.

Figure 2 side view of Winter Skate (L. ocellata). image credits: Space for Life Montreal

The lower surface is usually white with irregular light brown blotches. as shown in Figure 2.

Winter Skate, located in the Gulf of Saint Lawrence, matures at a much younger age and smaller size. L. ocellata resembles the Little Skate (L. erinacea). The estimated age at 50% maturity in this population is about five years.

Reproduction and development

Winter skates are oviparous, meaning that this species is egg-laying. Their reproductive strategy involves slow growth and late maturity. They lay one egg in a purse or capsule. On average, L. ocellata lays 50 eggs a year, while Little Skate lays 30 eggs. This implies the Winter Skate species has a higher fecundity than the Little Skate. The egg capsule is most like a bird or reptile shell. Gestation time for Winter Skates in the Gulf of Saint Lawrence is like the Little Skate at 6 to 9 months.

Development stages follow the ontology shown in Figure 1. These are commonly conserved stages among skates. However, there are differences in the timing of the reshaping of the mandibular arch region, the development of branchial filaments, median fins, denticles, and the order of the branchial cleft formation. These are taxon-specific, making the universal staging for chondrichthyans less applicable.

Stages 28-33 of Development

Figure 3 Development of skate-specific morphologies, including the mouth, pectoral fins and pelvic fins. Both sexes are illustrated for stages 30–32. Scale bar = 2 mm stages 28–31; 5 mm stages 32 and 33.

Stages 28 through 33 of development are shown in Figure 2.

Stage 28: The lens is clear, but the eyes remain lateral (on the sides) and unpigmented. The posterior part of the rostrum (nose portion) sinks into the mouth. The mouth becomes more oval. The rostrum forms a 90-degree angle with the horizontal. The gill filaments are long and vascularized. The pelvic fin is divided into anterior and posterior lobes (towards the head and the tail, respectively). Both dorsal fins are present but are symmetrical.

Stage 29: the rostrum is small and round and forms an 80-degree angle with anterior horizontal. The mouth is an oval shape and is beginning to form a rectangular shape. The iris is completely pigmented. The spiracle (breathing organ) is round and open. Gill filaments are vascularized at maximum length. The pectoral fins have extended to the level of gill arches 5 and 6 and are beginning to overlap the pelvic fins posteriorly. The posterior dorsal fin becomes more elongated.

Stage 30: The rostrum is rounded. The mouth becomes rectangular. The spiracle is rectangular. Gill arches become partially fused but remain free of the pectoral fin. The pectoral fin forms an anterior hood. The pelvic fin remains convex and later acquires hood in males. Skates can be sexed from this stage. There is an overlap between the paired fins. Rows of scales are present along the lateral surface of the body.

Stage 31: The rostrum is pointed and rotated, so it is ventrally flattened. Eyes begin to shift dorsally but do not yet rotate upward. The pupil has keyhole morphology. The mouth becomes slit-like, and external nares migrate anteriorly. The spiracle is located behind the eye and is directed laterodorsally. The anterior tip does not fuse to the head or extend past the eye yet. The gill arches completely fuse dorsally. The pectoral and pelvic fins increase overlap, and by the end of the stage, the pectoral fin overlaps one-third of the pelvic fin. The pelvic fin acquires its butterfly-wing morphology. The claspers (reproductive organs) are visible and separated from the posterior lobe. The stomach is swollen with yolk. Both dorsal fins are well-developed. Tissue from the body wall begins to invade the dorsal fins.

Stage 32: The eyes are now on the dorsal sides. The iris is pigmented black, and the rest of the eye is grayish. The spiracle is large, slit-like, and located directly behind the eye. The external gill filaments decrease throughout this stage. The pectoral fins have fused to the gill region and rostrum. In males, the claspers become a separate lobe of the pelvic fin. The dorsal fins are equal in size and form acute angles with the body. There are three rows of scales along the dorsal surface of the body. The epidermis becomes pigmented.

Stage 33: The adult pigmentation is acquired. All scales have erupted and are distinct. The external gill filaments have regressed. The pectoral fins are completely fused to the rostrum and end anteriorly in a pointed projection. The margins of pelvic fins appear fringed. The dorsal fins are large, and the acute angle has increased. The yolk sac is present, only decreasing slightly.

Distribution and habitat

This skate can be found in the northwest Atlantic Ocean, ranging from North of the Gulf of St. Lawrence in Canada to Cape Hatteras, North Carolina

Winter skates prefer sand and gravel habitats. They are primarily found in depths below 111 m and up to 371 m and in temperatures ranging between -1.2° and -15 °C.

A dried-up egg case of the winter skate on the shores of Newfoundland and Labrador in 2019.

The local populations of the winter skate are not all evenly distributed. L. ocellata is facing extirpation in the Southern Gulf of St. Lawrence, for example, resulting in a lot fewer egg cases found on local beaches. Their range has declined 99% from the 1980s to 2015 in that sector. These egg cases are commonly referred to as mermaid purses due to their shape.

Conservation status

L. ocellata is considered endangered by the IUCN (The International Union for Conservation of Nature), which is due to different human and natural occurrences, including fishing and predation.

Fishing

While there has been no directed fishing for skates in the Gulf of Saint Lawrence, L. ocellata are incidentally captured in commercial fisheries aiming to catch other fishes. Most individuals are discarded. L. ocellata are often caught in the scallop industry since their distributions overlap.^, From 1971 to 1972, the estimated discard was around 100 tons but decreased to around 70 tons in the mid-1970s, to 25 tons annually in the 1990s, and around 1 to 2 tons since 2005. Most incidental captured skates are discarded, so a discard mortality rate is needed to accurately estimate losses. Data regarding Winter Skate discards have only been available since 1991, so assumptions are needed to assess previous years.

Bottom-contacting mobile fishing gear can significantly alter their habitat, including the seafloor structure and relative abundance of benthic species L. ocellata prey on.

Natural Factors Limiting Survival

The GSL population decline is mostly determined by the elevated adult natural mortality.^, Gray seals are the largest contributor to adult mortality because of the steep rise in their population in the GSL. Due to changing temperatures, their ranges overlap, particularly during summer months.

Food and feeding habits

Figure 4 Graph of the fullness index for the south Gulf of St. Lawrence Winter Skate (all sizes) in Northumberland Strait from July to August 2002-2009. Hours 0600 and 2000 were excluded from statistical testing.

Winter skates consume a variety of organisms as prey throughout their lifetimes. When they are small juveniles, they consume mostly small shrimp, such as the seven-spine bay shrimp (Crangon septemspinosa) and gammarid amphipods, which are shrimp-like crustaceans. L. ocellata also consume a lot of sand lance (Ammodytes) when they are small.

Adult L. ocellata consume mostly fishes, including rainbow smelt (Osmerus mordax) and winter flounder (Pseudopleuronectes americanus). They also consume Atlantic rock crab (Cancer irroratus), Atlantic razor clams (Siliquia costata), Atlantic herring (Clupea harengus), three-spined stickleback (Gasterosteus aculeatus), and various flatfish (Pleuronectiformes).

Feeding patterns based on stomach fullness of samples determined that they are most full when they first sampled at 0600 hours and 2000 hours, meaning they are most likely feeding at night. The dips and peaks throughout the day are shown in Figure 4.

References

References