254 f^- S. WiMPENNY 



1933 the catch stabiUzed again at around 30 tons a year, and finally in 1934 and 1935 

 there were indications that a decline had set in once more. 



The catch per boat also shown in Fig. 1 corresponds to the catch, and indicates 

 that the latter is also a reasonable measure of the stock density in the lake. Consider- 

 ing the catch as a measure of the stock, we may therefore try to account for the catch 

 changes just mentioned as stock changes, finding it profitable to examine these in 

 relation to the annual mean water levels shown in Fig. 1 . The water levels of such a 

 shallow lake as Mariut must be considered as of very great importance in deciding 

 the actual size of the habitat. The period of increase to 1923, due to fry introduction, 

 followed by the stability or slight increase to 1926, were years in which the lake level 

 oscillated between —2-75 and —2-90 m. In 1927, however, there was a sudden descent 

 of 12 cm in this level, and in 1928 and 1929 of another 5, bringing the lake to —2-99 m. 

 It is thought that this lowering of the lake made the fish more vulnerable to capture 

 in 1927, and so explained the great increase in availability of the fishable stock. By 

 1928 the effect had begun to wear off, and the Annual Fishery Report for that year 

 recording a failure of the whole fishery of the lake says, " This failure has not been 

 confined to one or two particular species but has been one of the organic production 

 of the lake itself. The usual rich growth of Ruppia maritima and floating algae was 

 very much diminished and in some parts and at some times absent." The lake level 

 returned to —2-94 again in 1930, but thereafter fell sharply away again to a new low 

 record of — 3-21m in 1933 and remained at — 3-18m for the last two years. 



The evidence on lake level, therefore, looks as if the fish stock had to make some 

 new adjustments to its environment — a critical lowering of the lake level which appears 

 to have begun around —2-94 or —2-95. 



Age, growth and maturity 



Samples taken from the lake at or around the spawning season in 1928-1931 and 

 in 1933 are given in Table II. Length measurements were made to the centimetre 

 below, and 0-5 cm added to allow for the distribution of length within the centimetre. 

 Age was determined by scale readings, it having been proved by direct observations in 

 ponds that a ring is laid down in the first scaled winter. The weights, direct in the case 

 of the 1933 sample, were derived from the lengths in the remaining samples, using an 

 age-length relation taken from samples of this species collected from Lake Mariut 

 and near the entrance to Lake Menzala in November 1931. 



The numbers at the different ages show that there are few that have been in the lake 

 three winters or more (III+ group), and that the majority of the fish that are leaving 

 or preparing to leave the lake to spawn have been therein for one winter after their 

 introduction, and are on the eve of their second. This group has increased its pro- 

 portion by weight during the period from 45 to nearly 60%, whilst that of the fish 

 of the O group that are not quite one year old has declined from 42 to 17%. This 

 relative decline of the O group in the spawning and pre-spawning shoals, seems most 

 likely to be due to successively smaller O group recruitment enhancing the importance 

 of the I group, which each represent an O group of superior strength in the previous 

 year. The estimates of the numerical strength of the O group given in Table I support 

 this suggestion; but it would be unwise to take these estimates too exactly, in view 

 of the many errors involved when using such small samples as are available to me. 



