Smith, Ronald Stuart Martin
The biology of larval and juvenile Nephrops Norvegicus (L.) in the Firth of Clyde.
PhD thesis, University of Glasgow.
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In Scotland over the past 30 years the Nephrops fishery has expanded and is now the largest in Europe with landings in 1986 valued at over 㴍m. Much of the species biology is poorly understood, especially the early life history. The biology of larval and juvenile Nephrops is the subject of this thesis and the work reported has been conducted on Nephrops within the Firth of Clyde. Because Nephrops incubate their eggs externally (on their pleopods) they suffer progressive egg loss. Creels were set to capture ovigerous females incubating eggs at various stages of development to provide fecundity data and egg samples for a study of the biochemical changes associated with embryonic development. During the incubation period mean egg loss was estimated as 18% . Almost a fifth of the population were judged to have suffered additional egg loss, probably at the moment of spawning. Estimation of the mean number of eggs hatched per female per year took such losses into account and involved the use of length frequency information and a maturity ogive. The best estimate of the fecundity range was 985 - 1115 eggs hatched per female, which includes an allowance for the proportion of the population that may be biennial spawners. Lipids are the principal energy reserve utilized during embryonic development and full development requires 6 - 7 joules per egg. Most energy is expended during the later stages of development and is associated with a rapid uptake of water and salts. An aquaculture facility was developed and newly hatched larvae were reared under different culture conditions. Survival was found to be better in isolation compared to mass rearing conditions and initial periods of starvation in excess of 1 -2 days led to an increase in mortality. Larvae were reared over a temperature range of 8 - 20oC and relationships were derived between temperature and the rate of larval development for each zoeal stage. The development time increased for successive zoeal stages at each temperature and 16oC seemed to be the overall `optimum' temperature for development. A larval survey programme was conducted in the Lower Firth of Clyde (L.F.C.) to investigate temporal and spatial changes in larval abundance. The larval abundance data were used in conjunction with the temperature information obtained in the laboratory study to obtain a seasonal production estimate for the 1st zoeal stage of 177 x 108 larvae within the 876 square kilometres sampling area. This value, when used in conjunction with the mean number of eggs hatched per female per year, gave an estimate of 15.87 x 106 to 17.97 x 106 females for the L.F.C. spawning stock. Information on the vertical distribution of Nephrops zoeae was obtained over two 24h cycles, one coincided with a spring and the other with a neap tidal period. The distribution of the larvae was related to several environmental variables and compared with the light and pressure responses demonstrated under controlled laboratory conditions, 1st, 2nd and early 3rd stage zoeae demonstrated positive phototaxis and high barokinesis. No dramatic diel changes were detected in the vertical distribution of the larvae. Most remained between 6 to 38m depth during daylight hours. A limited nocturnal ascent resulted in peak larval abundance shifting from 16 to 5m depth. Shortly after dawn the larvae descended to their daytime distribution. Depth regulation is probably achieved by light and pressure sensitivity and may be influenced by the position of the pycnocline. During the 3rd zoeal stage the light response changed from photopositive to photonegative and Nephrops may first come into contact with the sea bed towards the end of this stage. Substratum selection and settlement behaviour experiments were conducted with early postlarval stages and these suggested that the 1st postlarval stage is a transitional form between the planktonic and benthic environments.
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