A Preliminary Study on the Length Frequency Analysis for Estimation of the Growth Parameters of the North Sea Mackerel Scomber scombrus Linnaeus 1758

&nbsp;The growth parameters of the North Sea mackerel Scomber scombrus Linnaeus, 1758, like most other temperate species, are estimated&nbsp; from length at age data. Ageing of mackerel is based on otolith reading. In this study, it was attempted to apply the latest versions of computerized length frequency analysis in order to estimate the growth parameters of the North Sea mackerel and also to compare the&nbsp; results with those obtained using length at age data. The growth parameters, Loo and k values estimated by the length frequency analysis were considered very much acceptable when compared with the values from length at age data. The t0 value, however, could not be obtained through the length frequency analysis.</p

Near-term embryos and gravid females of Lusitanian cownose ray (Rhinoptera marginata) in Mersin Bay, eastern Mediterranean Sea

Batoid chondrichthyans are vulnerable to commercial fisheries even though they are often not targeted. A greater understanding of their reproductive biology is important to facilitate their conservation. In February 2013, 89 female and 40 male Lusitanian cownose ray (Rhinoptera marginata) specimens were accidentally caught in Mersin Bay in the eastern Mediterranean Sea. Thirty-six fish were gravid, each bearing one embryo. Of the remaining females, 39 were sexually mature but not gravid and 14 were immature. There were 28 mature and 12 immature males. The average disc width (DW) of both mature females (80.2cm) and males (74.2cm) was significantly higher than that of immature females (60.7cm) and males (60.3cm). For all rays, DW and total weight (TW) were in the range 55.7-98.2cm and 1802-13600g. The DW and TW measured for all 16 female and 20 male embryos were in the range 15.6-29.2cm and 57-281g. Such a large single haul sample, including many gravid specimens and mature males, indicates a schooling formation at capture. The location in Mersin Bay near the estuaries of the Tarsus and Seyhan rivers, and its relative closeness to the sampling location of a previously reported neonate, suggests that this region may be a nursery area for Lusitanian cownose rays in the eastern Mediterranean

An evaluation of the precision of diet description

Percentage compositions by weight and by number are the frequently used measures to evaluate the relative importance of different prey types in fish diet studies. The results obtained using these measures are only point estimates, and are often reported without any indication of their precision. Here an attempt is made to set confidence limits to these estimates via normal approximation and bootstrapping. Applications of these approaches are demonstrated with the stomach contents data of mackerel from the North Sea. The precision estimates of the average weight percentages were generally very poor, even when the prey items were grouped into major categories and sample sizes were comparatively large. Intra-haul correlation was found to be an important source of variation in the stomach content composition. A bootstrap method, which incorporated inter- and intra-haul variation, provided more realistic confidence intervals for weight percentages than the normal approximation. Implications of the uncertainty associated with these measures for dietary studies have been discussed. Routine estimation of the precision of dietary measures is recommended