35. Variations in the Catches of Nephrops norvégiens at Different Times of Day and Night

By

A. C. S im p s o n Fisheries Laboratory, Burnham-on-Crouch

Introduction trawls with 82 ft bass groundropes wound with chain. Changes in the behaviour of Nephrops during each The cotton net used in July 1962 was a Gundry prawn 24-hour period, which result in marked variation in trawl with a 120 ft groundrope with 140 two-ounce catchability, are important in determining the best lead rings in place of the chain. way of catching them and in influencing the size of In all cases except in the North Sea, where no cover the catches during quantitative surveys. was used, the upper surface of the cod-end of the During investigations into the biology of Nephrops trawl was covered with loose fitting shrimp netting, and the distribution and size composition of Nephrops and the numbers of Nephrops plotted in Figures 1-6 stocks in different areas, several series of observations refer to total catches of males and females of all sizes have been made which throw light on the diurnal in the net and cover together. In these diagrams the variation in their catchability. The earliest of these vertical scales have been chosen to give similar sized series (Irish Sea, 2.-4. September 1960) was described curves for ease of comparison. The materials used in by S im p s o n (1961), and the present account includes the cod-ends are indicated by the use of different part of those observations and a number of later ones. symbols (open circles = sisal, solid circles = courlene, Whilst each series of observations was completed in + and x = cotton). the same position with the same gear, a variety of gear was used in the different series. The details of the gear Results used, the dates and the depths of water are summarized The observations do not cover all seasons nor all in Table 1. depths at which Nephrops are found and therefore the The sisal net was a standard Lowestoft trawl with a pattern of catches obtained may not be complete. 126 ft combination groundrope carrying 31/2 inch In all the series of observations the catches were rubber discs. The courlene nets were Gundry prawn highest just after daybreak and just before it became

Table 1. Summary of details of N ep h ro p s experim ents, 1960, 1961 and 1962

Mesh Mesh of D uration Depth Area Date Material of net of net cod-end of hauls Figure (fm) (mm) (mm) (his)

Irish S ea...... 30-32 26.-27. April 1962 Sisal 70 70 1 1 2.-6. July 1962 (a) Cotton 42 2 Irish S ea...... 11-19 Wings 63 lV r-21/« Irish S ea...... 11-19 2.—6. July 1962 (b) Cotton 70 2 Belly 50 1V2-2V 2 Irish S ea...... 11-19 2.-6. July 1962 (c) Cotton 70* IV s- 2 1/* 2 Irish S ea...... 30-32 7.-8. Sept. 1962 Courlene 70 70 1 3 Irish S ea...... 30-32 2.-4. Sept. 1960 Sisal 70 70 1 3 North Minch 64-69 17.-18. Sept. 1961 Sisal 70 66 1 4 South Minch...... 75-85 11.-12. Sept. 1961 Sisal 70 66 1 0 North Sea...... 38 18.-23. Sept. 1961 (a) Courlene \ Wings 70 50 2 6 N orth Sea...... 38 18.-23. Sept. 1961 (b) Courlene ) Belly 55 70 2 6

* Courlene cod-end. Hundreds per hour 3 Thousands per hour Thousands per hour ure 3. Irish Sea, 2.-4. September 1960 and 7.-8. September September 7.-8. and 1960 September 2.-4. Sea, Irish 3. ure iue5 SuhMnh 1.1.Spebr 1961. September 11.-12. Minch, South 5. Figure iue . rs e, 6-7 pi 1962. April 26.-27. Sea, Irish 1. Figure n —i —; —i —i— i— i— i— ;— i— i— i— — ON 18 NOON NOON 1962. Figures 1-6. Diurnal variations in catches of of catches in variations Diurnal 1-6. Figures i— * • o bridles No o ET 30- fm 2 -3 0 3 DEPTH No bridlesNo bridles fm 20 I960 1962 4 GMT 24 T M G 4 2 32fm 2 -3 0 3 187 iue6 NrhSa ofNrhSils, 8-3 etme 1961. September 18.-23. Shields), North (off Sea North 6. Figure f- -C TD o if) =j a C in C

D iscussion Résumé of discussion on the paper

A n d e r s e n (1960) and T h o m a s (1960) both referred Dr. C o l e , referring to the previous suggestion by to the changes in catchability of Nephrops at different Dr. J e n s e n that there was a difference in size com­ times of the 24-hour period. A n d e r s e n stressed the position of Norway lobsters taken by day and by sentivity of Nephrops to light and deduced from his night, asked if this had been found in the present observations that they tend to hide in the mud during experiments. Mr. S im p s o n said there was a very slight the day and to be active, feeding on the surface of increase in the proportion of the “smalls” category in the mud, during the night. He also considered that night hauls, but for all practical purposes the size the larger animals were less sensitive to light and are composition had been constant. Dr. J e n s e n , taking 189 up the point, said that off the south-west Swedish waters. Dr. T h o m a s replied that fishermen worked coast the day hauls had given a larger mean size of on the expectation of biggest catches at dawn and Norway lobster than the night hauls on the same dusk. Despite this, however, it had not shown up in ground. He added that a light-meter would be very the Scottish research fishings. Dr. T h o m a s agreed useful in investigating the diurnal variation in avail­ with M r. S im p s o n that the effect was probably less ability of Nephrops. This was generally agreed. marked in deep water and drew attention to other M r. S im p s o n referred to a paper submitted at a influences on availability, notably turbidity (which previous ICES Shellfish Committee meeting (1960) was associated with light), and also the effect of in which A n d e r s e n had stated that in Faroese waters ground swell and storms. catches were highest at night. He pointed out that at Professor N ik o l s k y asked if there was any correla­ the season when this work was done there was effectiv­ tion between availability and phases of the moon. ely no total darkness around Faroe, so that these Mr. S im p s o n said he had no evidence on this as yet. results could be comparable with the peak dawn/dusk Mr. S ig u r d s s o n reported the custom, in Iceland, of landings reported in his (Mr. S im p s o n ’s) paper. fishing more by night than by day. He realized that, Dr. W e n t reported that Irish workers had obtained at this latitude, even more so than at Faroe, there was similar results to those reported in the paper under effectively no total darkness in summer when the discussion. Norway lobster fishery was in progress. Icelandic Dr. C o l e enquired as to the findings in Scottish fishermen considered tides to be important.