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Fisheries Monitoring and Assessment > Nephrops norvegius biology and fishery > Flashcards

Nephrops norvegius biology and fishery Flashcards

1
Q

What class do Dublin bay prawns belong to?

A

Crustacea

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2
Q

What family do Dublin bay prawns belong to?

A

Nephropidae

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3
Q

What is the species name for nephrops?

A

Norvegicus

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4
Q

What names are there for nephrops?

A

Dublin bay prawns, Norway lobster, scampi, La langoistine

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5
Q

Describe the appearance of nephrops.

A

-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

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6
Q

How many pairs of legs have claws?

A

First three pairs of legs have claws

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7
Q

Describe the body of nephrops

A

-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

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8
Q

What is carapace length?

A

Parallel to the midline, from the back of either eye socket to the distal edge of the carapace

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9
Q

How is total length measured?

A

Tip of the rostrum to the rear end of the telson, not including setae

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10
Q

What is the distribution of nephrops norvegius?

A

-Restricted to North East Atlantic
-Coast from Morocco to Northern Norway and Iceland
-Mediterranean sea and Adriatic

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11
Q

What areas are absent of nephrops norvegius?

A

Black and Baltic Seas

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12
Q

What factors determine habitat suitability for nephrops norvegius?

A

-Substrate type
-Mean annual bottom temperature
-Salinity
-Oxygen
-Mean depth and mean annual surface chlorophyll

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13
Q

What depths are nephrops norvegius found at?

A

200m - 800m

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14
Q

What depth range is nephrops norvegius most abundant at?

A

40 -400 m

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15
Q

What habitat is nephrops restricted to?

A

To sticky mud sea bottoms in which it can borrow
(>40% silt and clay)

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16
Q

Describe a nephrops burrow

A

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

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17
Q

What is the purpose of the burrow?

A

Provides protection from predators like cod

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18
Q

When do nephrops leave their burrows?

A

Depends on light intensity, tidal cycles, season (moulting) and reproductive status
-Mainly leave burrow at dawn and dusk

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19
Q

What factors affect leaving the burrow?

A

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

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20
Q

Are nephrops solitary animals?

A

Yes, however multiple occupancy can occur in the burrows

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21
Q

Where do nephrops spend the majority of their time?

A

Either lying in their burrows or by the entrance, only leaving shelter to forage or mate

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22
Q

What do nephrops feed on?

A

-Polychaetes
-Crustacea
-Mollusks
-Echinoderms
-Occasionally small fish

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23
Q

What does the larval nephrops feed on?

A

Feed on planktonic crustacea

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24
Q

What does CL stand for?

A

Carapace Length

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25
Q

What does TL stand for?

A

Total length

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26
Q

What age are female nephrops sexually mature?

A

2 years

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27
Q

What size are female nephrops when they are sexually mature?

A

23 mm CL and 70 mm TL

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28
Q

What age is male nephrops sexually mature?

A

3 years

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29
Q

What size is male nephrops when sexually mature?

A

26mm CL and 85mm TL

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30
Q

What is the nephrops reproductive cycle linked to?

A

Latitude and temperature

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31
Q

How often do nephrops reproduce in cold waters off Norway and Iceland?

A

Biennially

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32
Q

How often do nephrops reproduce?

A

Annually

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33
Q

How many eggs do nephrops produce?

A

400 - 600 eggs depending on size

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34
Q

How long are fertilised eggs carried by the female?

A

9 months until they are released in the spring

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35
Q

When do nephrops reproduce?

A

In spring, between a hard-shelled male and recently moulted, soft-shelled female

36
Q

What does the female do after mating?

A

Stores sperm under her tail until late summer/early autumn, than releases eggs and fertilises them with the sperm

37
Q

How long do hatched larvae remain in the plankton?

A

3-4 weeks and stay in the area of release

38
Q

What happens after first 3 moults?

A

A bottom living juvenile (15-17mm) is produced

39
Q

What happens to the female after her eggs hatched?

A

She moults again and mates

40
Q

How do nephrops grow?

A

They grow by repeated moulting

41
Q

How much do nephrops grow in their first year?

A

30 - 50 mm

42
Q

How much to nephrops increase in length with each moult?

A

About 10% increase

43
Q

How many times do mature females moult a year?

A

Once a year

44
Q

How many times do mature males moult a year?

A

Twice a year, males grow quicker and get bigger than females

45
Q

What is the max size of females?

A

73mm CL and 230mm TL

46
Q

What is the max size of males?

A

93mm Cl and 290 mm TL

47
Q

What is the maximum weight of nephrops?

A

400 grams

48
Q

What size are nephrops by three years?

A

85 mm

49
Q

What is the age range of nephrops?

A

7 - 10 years

50
Q

Do nephrops from different areas vary?

A

They grow at different rates and mature at different sizes

51
Q

What is thought to cause geographic variation?

A

Density dependent
Food availability
Sediment type

52
Q

What is nephrops density like on soft mud?

A

Density is low, but animals grow relatively fast and reach a larger maximum size

53
Q

What is nephrops density like on sandier mud?

A

Density is much higher, but animals grow relatively slowly and are smaller

54
Q

What factors influence nephrops density?

A

-Nature of the substratum and its suitability for burrowing
-Patchy settlement of larvae
-Different levels of fishing effort / intensity

55
Q

What is the density of burrows in the North sea?

A

68 burrows / 100m2

56
Q

What low burrow densities can occur?

A

1 burrow / 4m2 has been recorded

57
Q

What are nephrops known to do in the winter?

A

Move to deeper waters

58
Q

How many European nephrop populations are there?

A

30 individual populations
-Separated by inhospitable terrain and adults rarely migrate/travel distances greater than a few hundred meters

59
Q

What predators predate on nephrops?

A

-Heavily predated on by cod in March when they spawn in NW Irish sea
-Thornback rays
-Dogfish

60
Q

What parasites affect nephrops?

A

-Epizotic parasites / diseases common in animals that don’t moult frequently
-Haematodinium (parasitic dinoflagellate infection)
-Shell and muscle necrosis
-Blackspot or melanosis

61
Q

What is the most important commercial species in Ireland?

A

-Nephrops most important commercial crustacean in Ireland and Europe
-Next to mackerel, Nephrops are the second most valuable species in Ireland

62
Q

What is the value of nephrops landings?

A

€37 m
-Irish landings support important onshore processing industry

63
Q

What % of EU landings of nephrops are landed in Ireland

A

40% is from Irish fisheries

64
Q

How are nephrops fished?

A

-Single, twin and multi-rig otter trawls
-Square mesh panel: 100 mm

65
Q

What is the mesh size for nephrops?

A

70 - 99mm

66
Q

How many vessels land nephrops?

A

approx. 48

67
Q

What countries fish nephrops?

A

-Ireland
-France
-UK
-Spain

68
Q

Where is nephrops fished in Ireland?

A

-Primarily Irish Sea 40% of total Irish catch landings
-Howth, Skerries, Clogherhead
-Celtic Sea
-North of the Aran Islands
-Porcupine Bank

69
Q

What are some mitigation methods for nephrops fishery?

A

-Square mesh panels
-Swedish grids
-Found to decrease by-catch

70
Q

Describe nephrops creels

A

-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

71
Q

Describe nephrops creel fishery

A

-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

72
Q

Advantages of Creel fishery?

A

-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

73
Q

How is age of neprhops calculated?

A

Length is used to calculate age using the von Bertalanffy growth model

74
Q

What is the recognized standard measurement for nephrops?

A

Carapace Length (CL)

75
Q

Describe nephrops stock assessment

A

-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

76
Q

How are nephrops fishing grounds managed?

A

Managed as Functional Units (FU)

77
Q

How are nephrops surveyed?

A

Underwater television surveys are undertaken to independently estimate abundance, distribution and stock sizes for fishing grounds

78
Q

What are some issues with the nephrops stock?

A

-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

79
Q

Describe the handling of nephrops post catch

A

-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

80
Q

What time of day produce greatest number of catches?

A

Timing of emergence - sensitive to wavelength 480 nm
-Greatest catches at dawn and dusk

81
Q

Are males or females more likely to be caught?

A

Males more likely
-More heavily exploited than females

82
Q

When are nephrops fished?

A

Bimodal fishery
-March - males
(Moult May/June)
July and August - females

83
Q

What nephrops grounds are video surveyed?

A

-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

84
Q

How are nephrops monitored?

A

-Annual Underwater TV Surveys (UWTV)
-Port sampling
-Trawl surveys
-Commercial fisheries assessment (CPUE)
-Discard sampling

85
Q

What data does Annual Underwater TV surveys collect?

A

-Using VMS data
-Density of burrows calculated

86
Q

How are underwater camera surveys carried out?

A

-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

Fisheries Monitoring and Assessment (10 decks)

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