How do dogfish move

Sharks tagged in the Netherlands have since been seen along the British, Belgium and French coasts and as far away as the Gulf of Biscay. Tagged sharks were even found in more northerly waters: north of Scotland and off the Norwegian coast. In , shrimp fishermen in the Wadden Sea began to tag sharks which they caught accidentally. They work together with the Waddenvereniging.

Lesser spotted dogfish are small bottom sharks and are harmless for man. Sometimes you find the empty olive-colored egg cases along the flood mark on the beach. You can usually see a tear where the young shark left the egg. The tips of the cases were once attached to a stone, shipwreck or maybe even seaweed. It is very unusual to find a case that still contains life. That is easy to check, since the egg capsule is transparent.

Lesser spotted dogfish are sometimes caught for consumption. However, dogfish are usually caught accidentally. Being a bottom dweller, fishermen catch them while going after flatfish. Because lesser spotted dogfish grow relatively slowly, there is a good chance that they are caught before reproducing.

Therefore, it is no wonder that lesser spotted dogfish have become more rare in intensively fished regions. Since the s, the lesser spotted dogfish has been the most common species of shark in the North Sea. To defend itself, the spiny dogfish may inject venom into predators from the two spines near the dorsal fins.

Humans are only at risk if they improperly handle these sharks. Based on evidence of over-exploitation in their range and bycatch fisheries, global population of spiny dogfish are considered vulnerable by the IUCN Red List. Global populations have declined by more than 30 percent over the last 75 years. In some parts of the world, this shark has been targeted for its meat and fins.

In , Oceana applauded steps taken by the Atlantic States Marine Fisheries Commission to prohibit the finning of spiny dogfish. Other fisheries take the spiny dogfish as unwanted bycatch before discarding them back to sea. Pacific spiny dogfish reach a maximum length of 4.

Because of their relatively small size, dogfish usually eat small fish, as well as jellyfish, clams, krill, octopus and squid. Like all sharks, dogfish have skin that is covered in tooth-like scales called denticles. But unlike most sharks, dogfish are also venomous.

They have two spines, one in front of each dorsal fin, that secrete a mild venom. These sharp spines serve as a defense mechanism against the dogfish's predators, such as sixgill sharks and seals. Dogfish are keystone predators, and ecologically important in their niche. Like many sharks, they have been historically misunderstood, as well as overfished. Currently, some of their populations are threatened due to a conflux of factors, including their natural predators, a lack of prey, and their popularity as seafood in Europe.

Overall, the mean longitudinal velocity was significantly higher for the second dorsal fin than for the first Table 1 , Fig. Mean lateral velocity and vorticity were not significantly different between dorsal fins, but were on average positive Fig. Velocity, longitudinal velocity, lateral velocity and vorticity behind the first dorsal fin in spiny dogfish, showing flow deceleration.

Stroke-averaged velocity and vorticity fields in the wake of the first dorsal fin in a spiny dogfish. A Velocity vector field. B Longitudinal velocity component.

C Lateral velocity component. D Vorticity fields. Inset in A: the location of the analyzed region on the shark body. The shark image represents the position of the shark at the beginning of the analyzed sequence. Velocity, longitudinal velocity, lateral velocity and vorticity fields behind the second dorsal fin in spiny dogfish, showing flow acceleration. Stroke-averaged velocity and vorticity fields behind the wake of the second dorsal fin in a spiny dogfish.

Averaged over the entire fin stroke, flow is less than free-stream velocity behind the second dorsal fin see Fig. Velocity and vorticity variables for both dorsal fins in four individual sharks of each species during four trials of steady swimming at each speed. Free-stream flow away from the shark was not different from that higher in the water column, indicating that we were well above the boundary layer.

Instantaneous flow behind each dorsal fin was accelerated in relation to incident flow and vorticity was present downstream of both dorsal fins Fig. Representative instantaneous velocity field around bamboo shark dorsal fins when the first dorsal fin is at maximal abduction to the right and the second dorsal fin is half-way on a right-to-left stroke.

Instantaneous longitudinal flow during the fin beat cycle in bamboo shark. Instantaneous longitudinal flow is accelerated in relation to free-stream flow behind the first A and second B dorsal fins throughout the fin beat cycle.

The dashed line represents mean incident flow to each fin. Increases in stroke-averaged longitudinal velocity were detected behind the two dorsal fins, with a strong lateral component in the opposite direction to fin movement Figs 9 and Strong vortices were also detected behind both dorsal fins Figs 9 and 10 , and were higher for the first dorsal fin.

When running two-way mixed model ANOVA with individual as a random effect and fin as a fixed effect, mean longitudinal velocity was similar between the dorsal fins Fig. However, it should be noted that the first dorsal fin was more variable than the second.

In contrast, mean lateral velocity was significantly higher behind the second dorsal fin Fig. Velocity, longitudinal velocity, lateral velocity and vorticity behind the first dorsal fin in bamboo sharks, showing flow acceleration. Stroke-averaged velocity and vorticity fields in the wake of the first dorsal fin in a bamboo shark. Velocity, longitudinal velocity, lateral velocity and vorticity behind the second dorsal fin in bamboo sharks, showing flow acceleration.

Stroke-averaged velocity and vorticity fields in the wake of the second dorsal fin in a bamboo shark. Comparison of bamboo shark and spiny dogfish dorsal fin function. Mean stroke-averaged velocity and vorticity values for both species' first dorsal fin D1 and second dorsal fin D2. Large circles represent the mean and error bars are standard deviation; circles and crosshairs represent individual trials.

Mean longitudinal velocity was different between fins and had interactive effects between species and fins Fig. Mean lateral velocity differed between species and fins Fig. Velocity and vorticity variables were generally higher for bamboo sharks than for spiny dogfish Fig. Also, there was a marked difference between the first dorsal fin in spiny dogfish, which is slowing down flow, and the second dorsal fin in spiny dogfish as well as both dorsal fins in bamboo sharks, which are overall accelerating flow Fig.

Our data show that the first and second dorsal fins of sharks have different hydrodynamic functions depending on the species.

Flow deceleration is likely related to a stabilizing function, while flow acceleration indicates thrust generation. Bamboo shark dorsal fins produce strong longitudinal velocity components with considerable lateral velocities that are higher for the second dorsal fin. In spiny dogfish, large lateral velocities in the direction of fin movement in addition to reduced longitudinal flow velocity relative to incident flow in the wake of the first dorsal fin support the initial hypothesis that this fin is functioning as a stabilizer, as it is not generating thrust by adding streamwise momentum.

This is to be expected as we have observed concurrent electromyographic activity in the left and right fin musculature Maia and Wilga, b. Flow incident to the second dorsal fin is strongly decelerated in the wake of the first dorsal fin in relation to free-stream flow.

A possible thrust function from this structure is supported by the instantaneous and stroke-averaged flow showing acceleration in the direction of fin movement Figs 3 and 5 , as well as by the peaks in instantaneous longitudinal velocity over a stroke Fig. Previous work on this species has shown that the dorsal fin oscillates in phase with the body although it has higher lateral displacement than the body Maia and Wilga, In addition, bilateral asynchronous muscle activity was observed, further corroborating that the second dorsal fin of spiny dogfish is not a passive structure Maia and Wilga, b.

The second dorsal fin in spiny dogfish may function to reduce the effect of the wake velocity decrement generated by the first dorsal fin. The high variability among trials, especially in the longitudinal velocity behind the second dorsal fin, also deserves to be mentioned. It is possible that spiny dogfish are using the second dorsal fin for small corrections in heading during steady swimming. Further research into how this structure is used during maneuvers and while swimming in unsteady flows could shed some additional light on a stability role for this fin.

In bamboo sharks, the longitudinal component of velocity for the first dorsal fin was greater than incident flow but less than the free-stream flow, indicating that the postcranial region of the bamboo shark decelerates flow Fig. Body tilting during steady horizontal locomotion is common in sharks Wilga and Lauder, , and hence flow passing over the head and moving toward the first dorsal fin is likely to be decelerated relative to the free-stream flow before reaching the first dorsal fin.

The high variability seen in the longitudinal velocity behind the fin might also be caused by the variable incident flow. The thrust component of the second dorsal fin is also higher than the incident flow.

Strong longitudinal components in the wake of the dorsal fin are also present in bluegill sunfish during steady swimming Drucker and Lauder, ; Tytell, In bluegill, the vortices shed in the wake of the dorsal fin interact and have additive effects to the tail vortices, which could also be happening for these two fins in sharks.

Interactions with the tail are not likely in bamboo sharks because of a larger spacing between the second dorsal fin and the tail Maia and Wilga, b and the subterminal shape of the tail's epicaudal lobe Wilga and Lauder, b. With this configuration, it is unlikely that the vortices shed at the second dorsal fin would encounter the vortices shed at the tail, and body tilting may place the tail at a more ventral position than the dorsal fins, allowing the fin wake to pass above the tail.

Lateral velocity components were present in the wake of both the first and second dorsal fin in both species. In general, the lateral velocity components were in the opposite direction to the moving fin, indicating that the water is moving from a high pressure region to a low pressure region, forming a jet in the opposite direction to fin movement.

The only exception is the first dorsal fin in spiny dogfish, where the flow moves in the same direction as the fin. The first dorsal fin is relatively stiff Maia and Wilga, a and the lack of fin bending during propulsion results in a different lateral velocity pattern and the lack of jet formation. In teleost fishes, fins functioning as active stabilizers beat laterally, producing large lateral forces, as shown by flow visualization data from the dorsal fin of brook trout Standen and Lauder, In spiny dogfish, for the first dorsal fin, the lateral velocity component is not higher than the downstream thrust component, unlike the pattern observed in trout.

In bluegill sunfish, significant lateral velocities are present in the wake of thrust-producing fins, although the measured lateral component was lower than the thrust component Drucker and Lauder, Lateral losses might be higher in sharks that use dorsal fins for propulsion when compared with actinopterygian fishes.

This could stem from the lack of active control over individual fin ray curvature in sharks, in contrast to active fin curvature control observed in actinopterygian fishes Standen and Lauder, ; Lauder, The relationship between supporting fin ray structures in fin, and patterns of hydrodynamic force generation is as yet not known, and represents an intriguing area for future study.