On the spatial and temporal characteristics of some pelagic resources of the Adriatic
Abstract
The results of an analysis of the horizontal distribution of the sardine, (Sardina pilchardus /W a l b./), sprat (Sprattus sprattus /L./), anchovy (Engraulis encrasicolus /L./), mackerel (Scomber scombrus L.) and Spanish mackerel (Scomber japonicus H o u t t.) in the Adriatic, and of their catches in the Yugoslav fishing area, as well as those dealing with the »stock units« and subpopulations are summarized. The results show a wide horizontal distribution of the sardine, anchovy and mackerel, a localized distribution of the sprat, and some discontinuity in the distribution of the sardine, anchovy and mackerel due to the Jabuka Pit (M u ž i n i ć, 1973, 1973a). The Yugoslav catch distribution also shows a rather wide distribution of the Spanish mackerel in the Adriatic (Figs. 1.1, 2.3.1 and 2.3.2).
The data of direct and indirect observations indicate a wide horizontal distribution of the sardine and anchovy during the spawning season too.
The results of an analysis of the sardine »stock units« in the Adriatic pointed to some heterogeneity of the presumed »stock unit« (L a r r a ñ e - t a, 1968).
From the summarized results on the sardine subpopulations a homogeneity of the Mid-Adriatic sardine population comes out (M u ž i n i ć, S., 1936; M u ž i n i ć, 1954) as well as a heterogeneity of the population in the northern Adriatic (F a g e, 1920; M u ž i n i ć, 1954; Z a v o d n i k, D., 1962; K r a j n o v i ć, 1968; K r a j n o v i ć - O z r e t i ć, 1971) and in the Albanian waters (M u ž i n i ć, 1954; R a k a j, 1959).
The distribution of the Yugoslav anchovy catch, in the 1958-1971 period, was similar to the distribution of the Italian anchovy catch in the Adriatic Fig. 2.3.3).1) Conversely, the share of the Yugoslav catch of the sardine and sprat, as well as of the mackerel and Spanish mackerel in the Mid-Adriatic was larger than the share of the Italian catch of these pairs of species.2) A larger share of the Italian catch of the mackerel and/or Spanish mackerel in the southern Adriatic points to a more intensive Italian fishing in deeper waters.
An analisys of the distribution of the Yugoslav sardine, anchovy, mackerel and Spanish mackerel catches throughout the year, in the 1947-1971 period, showed a gradual increase in the share of the sardine catch in autumn ( October); in the 1965-1971 period, a large increase in the share of the anchovy catch took also place, as well as a less marked increase in the share of the mackerel and Spanish mackerel catches (Fig. 3.1).
The distribution of the Yugoslav catch of the sardine, sprat, anchovy, mackerel and Spanish mackerel throughout the year had a rather seasonal character (Figs. 3.1, 3.4 and 3.5). Conversely, the distribution of the Italian catch of these species throughout the year was more uniform as shown by the data for the 1965-1971 period (Fig. 3.5); this was especially the case for the anchovy catch and to some extent for the mackerel and Spanish mackerel catch (Fig. 3.4), (an increase in the Italian anchovy catch in late autumn and winter started really in 1961).
The mean Yugoslav annual catch of the sardine, in the 1947-1971 period, was larger than the catches of the sprat and anchovy (these being similar to each other) and especially than the catch of the mackerel (the catch of the Spanish mackerel being insignificant), (Table 4.1.1).
The highest coefficients of variation were recorded in the Yugoslav annual catches of the anchovy and Spanish mackerel, in the 1947-1971 period; the lowest variation coefficient was found in the sardine and the intermediary values in the sprat and mackerel (Table 4.1.1).
The mean Yugoslav annual catch of the sardine and sprat, in the 1958-1971 period, exceeded the mean Italian catch of this pair of species (Tables 4.1.2 and 4.1.3). Conversely, the mean Yugoslav annual catch of the anchovy (3.34 thousand tons) was by far smaller than the mean Italian catch of this species (15.20 thousand tons). The mean Yugoslav annual catch of the mackerel and Spanish mackerel was also smaller than the Italian catch of this pair of species. While the sardine and sprat (taken together) dominated in the total Yugoslav catch of the studied species (on average 72.2%), the anchovy was dominant in the Italian total catch (on average 54.7%).
The mean Yugoslav annual catch of all the studied species, to 1951-1971 and 1958-1971 periods (15.56 and 17.88 thousand tons respectively), was much smaller than the mean Italian annual catch (26.04 and 27.79 thousand tons respectively), (Tables 4.1.2 and 4.1.3). The difference might be in part explained by the Italian preferential anchovy catch, winter fishing and more intensive fishing in deeper waters.
While the coefficients of variation in both the Yugoslav and Italian annual catches of the sardine and sprat (taken together), in the 1958-1971 period, were similar to each other, the variation coefficients of the Yugoslav annual catches of the anchovy and of the mackerel and Spanish mackerel (taken together) highly exceeded the values recorded in the Italian catches (Tables 4.1.2 and 4.1.3).
Tendencies to coincide and alternate in the fluctuations of the total Yugoslav annual catches of the studied species and of the catches in smaller areas, in the 1947-1971 period, were very frequent (Figs. 4.2.1-4.2.4). However, only in some cases the observed correlations had a statistical significance (Tables 4.2.1- 4.2.4). In the same pair of species, the opposite significant correlations in different smaller areas were also found (Tables 4.2.2 and 4.2.3).
Highly significant correlations were also recorded an the annual catch fluctuations of the same species (sardine, Spanish mackerel) for the adjacent smaller areas (Table 4.2.5, Fig. 4.2.5).
A difference in the trend between the Yugoslav annual catches of the sardine, sprat and anchovy on one hand and the mackerel and Spanish mackerel on the other hand was found in the 1947-1971 period (Table 4.3.1, Figs. 4.3.1, 4.3.2 and 4.3.3). Different trends were also recorded in the Italian annual catches of the studied species (1958-1971), (Table 4.3.3, Figs. 4.3.4, 4.3.5 and 4.3.6). Finally, differences in the trend appeared between the Yugoslav and Italian annual catches of the sardine and sprat (taken together), and the anchovy (1958-1971), (Tables 4.3.2 and 4.3.3, Figs. 4.3.4. and 4.3.5), (the difference between the regression lines for the sardine and sprat taken together was analysed by F-test and a high significance of the difference was recorded).
The upward trend in the Yugoslav annual catch of all the studied species, within the 1947-1971 period, was due to similar upward trends in the catches of the sardine, sprat and anchovy (Tables 4.3.1 and 4.3.2, Figs. 4.3.7, 4.3.8, 4.3.1 and 4.3.2). Conversely, the upward trend in the Italian catch was due to a marked upward trend in the anchovy catch (1958-1971), (Table 4.3.3, Figs. 4.3.8 and 4.3.5).
Several factors have been presumed to be responsible for the upward trends in the Yugoslav annual catch of the sardine, sprat and anchovy, and, consequently, of all the studied species, such as the expansion of the fishing area (starting in the more northern, shallower, Adriatic and then in deeper waters of the Central Adriatic), the extension of the fishing season to the autumn months and the gradual improvement in the fishing techniques and tactics.
A highly significant negative regression of the Yugoslav annual catch of all the studied species on the fishing intensity shown by the number of fishing days (b = - 0.96 thousand tons, t = - 4.51**) was found in the 1961-1971 period (Fig. 4.4.5). Such a relationship might have been determined by a more efficient fishing for pelagic fishes, due in part to an intensification of fishing in autumn, at the time of high aggregability of the sardine and possibly also of other species.
A highly significant positive regression of the Yugoslav annual catch of sardines in the open and channel waters of the Split area in wider sense on the catch in the last third of the year was found in the 1961-1971 period (b = 2.32 hundred tons, t = 9.09** and b = 2.40 hundred tons, t = 3.65** respectively), (Figs. 4.4.2 and 4.4.3.). It was the same with the total Yugoslav annual catch of sardines and the catch of all the studied species (b = 1.47 thousand tons, t = 6.21** and b = 0.98 thousand tons, t = 3.80** respectively), (Figs. 4.4.4. and 4.4.6).
The favourable effect of stronger sardine year-classes on the catch in the Split area in wider sense might have been reduced by an intensive commercial fishing for young sardines.
A preferential fishing for the anchovy, in addition to an increase in the fishing efficiency, seems to be in part responsible for an upward trend in the annual catch of all the studied species in the Italian pelagic fisheries in the 1951-1971 period.
A unique cause of an important upward trend in the annual catch of all the studied pelagic fishes in the Adriatic (the Albanian catch excluded) (Fig 4.3.9) could not be presumed.
________________________
1) The Italian division of the Adriatic was used for this comparaison (areas 3-7 were included in the Central Adriatic, Fig. 1.1).
2) In the Italian fishery statistics more detailed data are given only for the 1958-1971 period. However, the sardine and sprat are taken together, as well as the mackerel and Spanish mackerel.
