In the period half May-December 1972 investigations were done to the population-structure of Grey Mullet (Mugilidae) in the western part of the Waddensea. The main question was: is this population open or closed? For this purpose were studied among other things: capture statistics from 1960, the age-strcuture of the population, length-frequencies, maturation during the time of investigation and growth. Most frequently found in this area was C. labrosus.Each season there are two maxima of capture for both grown up and juvenile Grey mullet. The first one (grown up: May;juvenile: June) probably coincides with entering the second (both: October) with leaving the Waddensea. Relations exist between Grey Mullet in the W. Waddensea and in the waters of Zeeland and Zuidholland. This finds expression in a simultaneous increase of captures in both areas during the years 1965 to 1969. The age-structure (from examination of scales and otholiths in captures of two succeeding seasons (1971, 1972) strongly differs. Yearly returning strong age-classes are absent. In the length-freguencies of 1971 and 1972 strong length-classes only scarcely have been observed. In both seasons the maxima of the length-frequencies is at 37 cm fish-length. Length-classes below 25 cm are absent for the greater part. The latter two facts point to swarming off out of the population-centre after reaching a particular length. Increasing maturation in autumn indicates a spawning season from January to April. Eggs and yearclasses 2, 3 and 4 have not been observed in the western Waddensea; larvae only sporadically. According to these data the population of C. labrosus in the w. Waddensea is an open one. The population-centre and spawning places probably are not in or near the W. Waddensea but in a more southern area. This area can extend from the Channel to the waters of Zeeland. A situation of the population-centre more southern from the Channel seems to be unlikely because of faster growth rates of C. labrosus in that area.
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