The 'dilul' or 'thinning' project

In 1992, when the commercial lavnun fishery was in the doldrums, fishermen noticed massive concentrations of small sardines (Landau, 1993). The following year government bodies set up the ‘dilul’ or ‘thinning’ project, by which purse-seine fishermen were paid to remove and dump the undersized, valueless fish. The ostensible purpose of the project was improvement of water quality by conserving zooplankton, on the assumption that fish predation is a major cause in the decline of zooplankton biomass in recent years (see Mirogrex biomass for further discussion).

The ‘dilul’ was providential for research as well as for fishermen. While the 'windows’ of commercial nets allow lavnun under 13 cm to escape, specimens as small as 8 cm total length were retained by the 'dilul' nets.

The Kinneret Authority kept accurate records of catch and workdays, as summarized in the table below.

Growth studies were facilitated by a size-distribution gap between lavnun in the 'dilul' fishery and lavnun in the commercial stock (> 14 cm total length). This gap was due to recruitment failure in the 1980's (Mirogrex biomass). The figure below shows year-to-year size progression, confirming the slow- growth hypothesis of Landau (1991).

Catch and effort data for the 3 purse-seine units in the 'dilul' fishery 1995 -1999.

End-of-season data (April-June, 1998 & 1999) were excluded. Data for some months, 1995 - 1997 were not available. On the reported workdays, all or nearly all effort was directed at 'dilul' fishing.

Mean weight was calculated from size distribution and weight data (Landau, 1998).

The C/E data indicate a rise in lavnun biomass between 1995 and 1997, followed by a steep decline. The rise can be attributed to growth of the 1990/91 and 1991/92 broods, as well as the recruitment of younger cohorts. The decline was due to an epidemic of ectoparasites that increased mortality, especially among the larger specimens and reduced weight by an average of 10% in 1998 (see Mirogrex biomass, Figure entitled '1998 parasite epidemic').

Comparison of 'dilul' to commercial fishing, and stock estimate

While the 'dilul' was carried out in daylight hours only, commercial purse-seining is either in day or night hours, with better sardine catches taken at night, with lights. From 1969 to 1989 (when monthly catch & effort data ceased to be available) C/E was 0.7 - 3.3 tons per night's fishing, with the exception of the 1986/87 season, when commercial catch averaged 5.3 T/night (Landau, 1996). Smaller specimens are not represented in commercial catches, but in steady-state these fish would not constitute more than a third of the stock biomass, and in the mid-1980's specimens of less than 14 cm were rare.

The 1997 C/E of 9.7 T is about twice the mean C/E for the mid-1980's, when stock biomass was 15,000 - 20,000 T according to my estimates (Landau, 1991). Thus the peak biomass of 1997 was ~ 30,000 T (Landau, 1998) or 180 g /m² (see Figure entitled 'Biomass trends').

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