Nephrops norvegius biology and fishery Flashcards
What class do Dublin bay prawns belong to?
Crustacea
What family do Dublin bay prawns belong to?
Nephropidae
What is the species name for nephrops?
Norvegicus
What names are there for nephrops?
Dublin bay prawns, Norway lobster, scampi, La langoistine
Describe the appearance of nephrops.
-Similar in appearance to a small lobster
-Claws are long and thin, same length as body
-Characteristic pale orange colour with red patches
-Black, kidney shaped compound eyes
How many pairs of legs have claws?
First three pairs of legs have claws
Describe the body of nephrops
-Head and thorax are fused (cephalothorax)
-Abdomen is long and segmented, ending in a broad tail-fan
-Two pairs of antennae, the second pair is longer and thinner
What is carapace length?
Parallel to the midline, from the back of either eye socket to the distal edge of the carapace
How is total length measured?
Tip of the rostrum to the rear end of the telson, not including setae
What is the distribution of nephrops norvegius?
-Restricted to North East Atlantic
-Coast from Morocco to Northern Norway and Iceland
-Mediterranean sea and Adriatic
What areas are absent of nephrops norvegius?
Black and Baltic Seas
What factors determine habitat suitability for nephrops norvegius?
-Substrate type
-Mean annual bottom temperature
-Salinity
-Oxygen
-Mean depth and mean annual surface chlorophyll
What depths are nephrops norvegius found at?
200m - 800m
What depth range is nephrops norvegius most abundant at?
40 -400 m
What habitat is nephrops restricted to?
To sticky mud sea bottoms in which it can borrow
(>40% silt and clay)
Describe a nephrops burrow
They dig a horizontal burrow with an entrance at each end
-Semi-permanent and vary in size and structure
-20-30cm deep and distances of 50 -80 cm between front and back entrances
What is the purpose of the burrow?
Provides protection from predators like cod
When do nephrops leave their burrows?
Depends on light intensity, tidal cycles, season (moulting) and reproductive status
-Mainly leave burrow at dawn and dusk
What factors affect leaving the burrow?
Water depth
-Shallow waters, rarely leave the burrow, just have head and claws sticking out
-Deeper waters, more active and spend less time in burrow
-Berried females rarely leave their burrows
Are nephrops solitary animals?
Yes, however multiple occupancy can occur in the burrows
Where do nephrops spend the majority of their time?
Either lying in their burrows or by the entrance, only leaving shelter to forage or mate
What do nephrops feed on?
-Polychaetes
-Crustacea
-Mollusks
-Echinoderms
-Occasionally small fish
What does the larval nephrops feed on?
Feed on planktonic crustacea
What does CL stand for?
Carapace Length
What does TL stand for?
Total length
What age are female nephrops sexually mature?
2 years
What size are female nephrops when they are sexually mature?
23 mm CL and 70 mm TL
What age is male nephrops sexually mature?
3 years
What size is male nephrops when sexually mature?
26mm CL and 85mm TL
What is the nephrops reproductive cycle linked to?
Latitude and temperature
How often do nephrops reproduce in cold waters off Norway and Iceland?
Biennially
How often do nephrops reproduce?
Annually
How many eggs do nephrops produce?
400 - 600 eggs depending on size
How long are fertilised eggs carried by the female?
9 months until they are released in the spring
When do nephrops reproduce?
In spring, between a hard-shelled male and recently moulted, soft-shelled female
What does the female do after mating?
Stores sperm under her tail until late summer/early autumn, than releases eggs and fertilises them with the sperm
How long do hatched larvae remain in the plankton?
3-4 weeks and stay in the area of release
What happens after first 3 moults?
A bottom living juvenile (15-17mm) is produced
What happens to the female after her eggs hatched?
She moults again and mates
How do nephrops grow?
They grow by repeated moulting
How much do nephrops grow in their first year?
30 - 50 mm
How much to nephrops increase in length with each moult?
About 10% increase
How many times do mature females moult a year?
Once a year
How many times do mature males moult a year?
Twice a year, males grow quicker and get bigger than females
What is the max size of females?
73mm CL and 230mm TL
What is the max size of males?
93mm Cl and 290 mm TL
What is the maximum weight of nephrops?
400 grams
What size are nephrops by three years?
85 mm
What is the age range of nephrops?
7 - 10 years
Do nephrops from different areas vary?
They grow at different rates and mature at different sizes
What is thought to cause geographic variation?
Density dependent
Food availability
Sediment type
What is nephrops density like on soft mud?
Density is low, but animals grow relatively fast and reach a larger maximum size
What is nephrops density like on sandier mud?
Density is much higher, but animals grow relatively slowly and are smaller
What factors influence nephrops density?
-Nature of the substratum and its suitability for burrowing
-Patchy settlement of larvae
-Different levels of fishing effort / intensity
What is the density of burrows in the North sea?
68 burrows / 100m2
What low burrow densities can occur?
1 burrow / 4m2 has been recorded
What are nephrops known to do in the winter?
Move to deeper waters
How many European nephrop populations are there?
30 individual populations
-Separated by inhospitable terrain and adults rarely migrate/travel distances greater than a few hundred meters
What predators predate on nephrops?
-Heavily predated on by cod in March when they spawn in NW Irish sea
-Thornback rays
-Dogfish
What parasites affect nephrops?
-Epizotic parasites / diseases common in animals that don’t moult frequently
-Haematodinium (parasitic dinoflagellate infection)
-Shell and muscle necrosis
-Blackspot or melanosis
What is the most important commercial species in Ireland?
-Nephrops most important commercial crustacean in Ireland and Europe
-Next to mackerel, Nephrops are the second most valuable species in Ireland
What is the value of nephrops landings?
€37 m
-Irish landings support important onshore processing industry
What % of EU landings of nephrops are landed in Ireland
40% is from Irish fisheries
How are nephrops fished?
-Single, twin and multi-rig otter trawls
-Square mesh panel: 100 mm
What is the mesh size for nephrops?
70 - 99mm
How many vessels land nephrops?
approx. 48
What countries fish nephrops?
-Ireland
-France
-UK
-Spain
Where is nephrops fished in Ireland?
-Primarily Irish Sea 40% of total Irish catch landings
-Howth, Skerries, Clogherhead
-Celtic Sea
-North of the Aran Islands
-Porcupine Bank
What are some mitigation methods for nephrops fishery?
-Square mesh panels
-Swedish grids
-Found to decrease by-catch
Describe nephrops creels
-Highly selective method of fishing- catch is brought up alive, unwanted animals can be returned to sea
-One ore more funnel style entrance making it easy for shellfish to get in but very difficult to escape again
-Can be used where nephrops grounds lie amongst rocks eg. Killary Harbour
Describe nephrops creel fishery
-Fleets of 40 -50
-Fresh bait is used
-Pots are inspected and re-laid daily
-Better size and condition of prawn caught by this method yield prices three - four times higher than animals netted by trawling
Advantages of Creel fishery?
-Better quality
-Reduced impact on seafloor
-Lower fuel consumption
-Allows fishermen with smaller boats to participate in high-value fishery
-Pots, traps and creels are exempted from the Landing Obligation
How is age of neprhops calculated?
Length is used to calculate age using the von Bertalanffy growth model
What is the recognized standard measurement for nephrops?
Carapace Length (CL)
Describe nephrops stock assessment
-No annually marked structures like otoliths in fish
-Stock size assessment and growth rate can therefore be difficult to determine since aging of Nephrops is not possible
How are nephrops fishing grounds managed?
Managed as Functional Units (FU)
How are nephrops surveyed?
Underwater television surveys are undertaken to independently estimate abundance, distribution and stock sizes for fishing grounds
What are some issues with the nephrops stock?
-Nephrops are discarded either because they are undersize or poor quality (poor moult)
-By-catch levels can be high ( 35 - 75%)
-Undersize nephrops
-Commercial fish - major discarding of whiting and some hake in the past
-Trawling has a severe negative effect on muddy seabeds and rock substrates
Describe the handling of nephrops post catch
-Prawns are iced and sold either frozen whole or as tails
-Head is removed as the digestive gland taints the flesh once dead
-Metabisulphate is added to prawn to preserve them
-Prawns are graded - larger prawns much higher prices
What time of day produce greatest number of catches?
Timing of emergence - sensitive to wavelength 480 nm
-Greatest catches at dawn and dusk
Are males or females more likely to be caught?
Males more likely
-More heavily exploited than females
When are nephrops fished?
Bimodal fishery
-March - males
(Moult May/June)
July and August - females
What nephrops grounds are video surveyed?
-West of Scotland FU 11 -13
-Eastern Irish Sea FU 14
-Western Irish Sea FU 15
-Porcupine Bank FU 16
-Aran, Galway Bay, Slyne Head FU 17-18
-South and South west of Ireland FU 19
-Labadie, Jones and Cockburn FU20-21
-Smalls Nephrops grounds FU22
How are nephrops monitored?
-Annual Underwater TV Surveys (UWTV)
-Port sampling
-Trawl surveys
-Commercial fisheries assessment (CPUE)
-Discard sampling
What data does Annual Underwater TV surveys collect?
-Using VMS data
-Density of burrows calculated
How are underwater camera surveys carried out?
-Under water video camera is towed over the sea bed for 10 mins travelling approx. 200m at 0.8 knots on a purpose built sledge
-Number of burrows are counted
-Burrow density estimates are the calculated for the area using statistical methods
-MI carry out surveys in summer months around Ireland
