ALBERT

Description: in a recent publication von Brandt (1) gives a survey of all determinations of the calcium content of Baltic water hitherto published. He records in all 39 analyses made during the last century, which give us an idea of the order of magnitude of the calcium concentration; they cannot, however, serve for comparative purposes as in many cases chlorine determinations on the same water samples are lacking. Neither have we any clue for judging the accuracy of these analyses, the latest of which date from 1884. Fifty years later, in 1935, Za rin s and O z o 1 ins (8) published an extensive investi­gation of the water in the Bay of Riga and in the Baltic off the Latvian coast, their most westerly station nearly coinciding with the Finnish station F81 (Lat. 57° 22'N., Long. 19°57'E.) above the central depression of the Baltic. Their material comprised about 70 calcium analyses on water from all depths. Finally v o n B r a n d t in the above-mentioned paper publishes nearly 300 analyses of surface water collected in 1935 and 1936 during several voyages from Pillau to Helsingfors and back, and along the German coast as far as Kiel and back. The present material comprises analyses of only 48 samples of surface and bottom water collected during the summer cruise, in July 1935, of the s.s. "Nautilus" from the Gulf of Finland, the Gulf of Bothnia, and the northern half of the Baltic proper. In spite of the smaller number of samples this material is more comprehensive than the two preceding investigations in so far as it covers a greater area of the sea. I t was originally meant as a survey of the calcium content in these parts of the Baltic, but the surprisingly simple relationships between calcium content and chlorinity which it revealed, give the results far more scope than was expected.

Description: The use of marine parasites as non-intrusive natural tags of their hosts was first broadly applied in fisheries science in the 1940s. Both micro- and macroparasites have been used to assess the status of current stocks of several commercially exploited species of marine animals. Here, Santiago Pascual and Eric Hochberg offer a brief comment on marine parasite tags as a stock assessment methodology, with special reference to cephalopod hosts.

Description: Male cuttlefish adopt a specific body pattern during agonistic behaviour called the Intense Zebra Display. Some components of the Display were variable, especially the chromatic component termed ‘dark face’, which could vary in the degree of darkness. Facial darkness was measured using a video analysis system. Males that eventually withdrew from conspecifics without fighting maintained a lighter face during the initial stage of agonistic encounters. When both males maintained dark faces, physical contact and fighting ensued. Therefore facial darkness could be used to predict which male–male encounters would escalate to physical contact. The strong correlation between facial darkness and subsequent behaviour suggested that males were signalling their agonistic motivation at the early stages of the encounter, which is contrary to what would be predicted from a traditional game theory analysis. It is proposed that males signal intent because the Intense Zebra Display simultaneously serves two functions: (1) it identifies the signaller as male, thus preventing unwanted copulations from other males, and (2) it functions as part of the agonistic behavioural repertoire. By using a modified (i.e. lighter-faced) version of the Display, males may be able to signal their sex, but without inducing another male to attack. In cases in which agonistic displays perform more than one function, signalling intent (i.e. signalling its likely subsequent behaviour) can be an evolutionarily stable strategy.

Description: A longline fishery for Dissostichus eleginoides has recently developed in the vicinity of South Georgia and Kerguelen islands, two internationally important breeding areas for procellariiform birds. The number of hooked birds and a method to reduce mortality were investigated during 13 days of fishing activity in Kerguelen waters in February 1994. Between 100 and 600 seabirds were always observed behind the longline vessel during daytime. The main ship-following species were the white-chinned petrel Procellaria aequinoctialis (67% of counts), giant petrels Macronectes spp. (8%) and the wandering albatross Diomedea exulans (11%), black-browed albatross D. melanophris (6%) and grey-headed albatross D. chrysostoma (2%). Only diving species were caught on the lines, i.e. the white-chinned petrel (n = 36) and the grey-headed albatross (n = 2). Marked differences in the mortality rate were observed between day and night (1·00 versus 0·38 birds per 1000 hooks), and at night when the decklights were on or off (0·59 versus 0·15 birds per 1000 hooks). Dumping of homogenized offal during line settings greatly reduced incidental capture of seabirds, mainly because birds were more attracted by offal than by hooked baits. We therefore propose that the dumping of offal during line settings should be included in the regulations of the longline fishery for Dissostichus in order to minimize seabird mortality.

Description: Assessment methodology for squid fisheries is presented, extending previous work by considering migration between adjacent fishing grounds. The methods are based on standard Leslie-Delury analysis but make different assumptions about stock movement and the relation between stock abundance and catch per unit effort. The new methodology is applied to data from the Illex argentinus fishery in the Southwest Atlantic, south of 45 °S. Retrospective assessments for the 1987–1991 fishing seasons are presented, focusing on estimates of recruitment and spawning biomass. Management of the fishery around the Falkland Islands is based on effort control. The objective is to maintain the spawning biomass above a threshold level, thus avoiding high probabilities of low recruitment in the following season. The estimates of spawning biomass and recruitment from the analyses are used to estimate an appropriate threshold level of spawning biomass.

Description: Samples of Sepia officinalis, S. orbignyana, and S. elegans from Galician waters (NW Iberian Peninsula) were obtained from commercial catches in three fishing ports. A preliminary screening for 47 and 33 enzymes in mantle muscle and digestive gland, respectively, was carried out using seven buffer systems. Thirty-seven enzyme loci were resolved from these tissues using only two buffer systems. Most enzymes shpwed equal or higher activity for the digestive gland than for the mantle muscle in freshly-caught samples of S. officinalis and S. orbignyana. The activity of a large number of enzymes decreased faster in the digestive gland than in the mantle muscle after 6-12 h at room temperature. Consequently, we suggest that mantle muscle rather than digestive gland be used for routine electrophoretic studies in Sepia species obtained from commercial catches. A phylogenetic reconstruction analysis, applying the Wagner parsimony method and using the ommastrephid species Illex coindetii as outgroup, showed only one most parsiminious tree. S. orbigyana and S. elegans were found to be a sister group. The topology agreed with that recently obtained from mitochondrial rDNA sequences, and both molecular data are in line with previous morphological results. They confirm the view that S. orbignyana and S. elegans belong to a different subgenus (Rhombosepion) from that of S. officinalis (Sepia "sensu stricto").

Description: Statolith microstructure was studied in two abundant planktonic cranchiids, Cranchia scabra (56 specimens, 38-127mm mantle lengh, ML) and Liocranchia reinhardti (34 specimens, 99-205mm ML) sampled in epipelagic waters of the western part of the Gulf of Guinea (tropical Atlantic). Growth increments were revealed in ground statoliths of both species. It was possible to distinguish two growth zones in statolith microstructure by their colour in reflected light of the microscope: the translucent postnuclear zone and pale white opaque zone. Assuming that growth increments in statoliths were produced daily, ages of the largest immature C. scabra and L. reinhardti were 166 and 146 days, respectively. Both cranchiids are fast-growing squids with growth rates in length resembling those of juveniles of tropical ommastrephids and Thysanoteuthis rhombus. Liocranchia reinhardti grows faster: its growth rate in ML is approximately twice that of same-aged C. scabra. The life cycle of both cranchiids consists of two phases. During their epipelagic phase, C. scabra and L. reinhardti feed and grow rapidly from paralarvae to immature young in the epipelagic waters, attaining 120-130 and 170-200 mm ML by ages of 4-5 months, respectively. Then they change their life style to a deepwater phase.

Description: Age composition and growth rates of the squid Loligo vulgaris (Lamark, 1797) were studied by examination of growth increments within statoliths of 419 specimens (mantle length, ML, ranging from 32 to 400mm). The squid were obtained by monthly sampling from the catches of commercial trawls off southern Portugal between March and September, 1993. The total number of growth increments in the mounted and ground statoliths was counted using a semi-automatic image analysis system. ML was significantly correlated with both the statolith length, TSL and the number of increments, NI. The female statolith was slightly larger than the male statolith for the same mantle size. Growth rates of individuals showed high variability with an average estimated at 34.6mm month-1 for males and 33.5mm month-1 for females. Growth in length between 70 and 280 days was best described by a power function for both sexes. The growth index of the statolith (TSL/NI) decreased with individual growth. The result may be related with the onset of sexual maturation. L. vulgaris hatched throughout the year with two distinct peaks, in spring which is the main breeding period, and in autumn. The life cycle of the L. vulgaris population on the south Portugese shelf was completed in one year.

Description: The planctonic life in Mediterranean Octopus vulgaris lasts about 2 months but we know virtually nothing of this phase of its life history, which represents around 10-15% of the estimated life span. Swimming behaviour from hatching to settlement was studied by video-recording techniques, using five groups aged 1,15,30,42 and 60 days, by when they have become benthic. During the planctonic stage, the backwards, squid.like jet swimming was the predominant type of displacement. Strong morphometric changes, basically in arm growth, influence their jetting capacities and probably the settlement process. Feeding behaviour was analyzed using two species of decapod zoeae as prey, Liocarcinus depurator (L.) and Pagurus prideaux Leach; it is that of a visual predator. The forward displacement typically forms part of this predatory behaviour. During the planctonic phase, the presence of prey increase the turning rate and reduces the swimming speed of Octopus vulgaris individuals. Both responses may improve the exploitation of patchy food environments.