A peaked logistic-based selection curve

Abstract

Length-based selection curves define the relative catchability of fish to specific types of fishing gear, with catchability often highest at intermediate fish lengths. Distributions such as the normal, lognormal, or gamma are often used to define "peaked" selection curves, but these have limited capabilities to describe strongly asymmetric selection relationships, such as those sometimes observed for hooks or gillnets. Another, more flexible, peaked selection curve is proposed, which is derived by combining multiple logistic distributions. While the logistic distribution is frequently used to describe monotonic selection curves, incorporating multiple logistic equations (that describe either the increasing or decreasing catchability) can define a large range of asymmetric peaked selection curves. This "peak-logistic" curve also allows nonzero asymptotic selection for the largest size classes, which may be the selection occurring in some hook-and-line fisheries. We demonstrate examples of selection in hook, haul net, and mixed hook fisheries, for which the peak-logistic curve is more appropriate than comparative lognormal and binormal selection curves. We also promote an alternative to the peak-logistic: the two-sided normal curve.Résumé : Les courbes de sélection en fonction de la longueur définissent la capturabilité relative des poissons pour des types donnés d'engins de pêche, cette capturabilité étant souvent à son plus fort pour des longueurs de poisson intermédiaires. Des distributions de types normal, log-normal ou gamma sont souvent utilisées pour définir des courbes de sélection « à pic », mais celles-ci se prêtent mal à la description de relations de sélection très asymétriques, comme celles qui sont parfois observées pour la pêche à la ligne ou au filet maillant. Une courbe de sélection à pic plus versatile est proposée qui est obtenue en combinant des distributions logistiques multiples. Si la distribution logistique est fréquemment utilisée pour décrire des courbes de sélection monotones, l'intégration d'équations logistiques multiples (qui décrivent une capturabilité soit croissante ou décroissante) peut permettre de définir une grande gamme de courbes de sélection avec pic asymétriques. Cette courbe « logistique à pic » permet également une sélection asymptotique ne tendant pas vers zéro pour les plus grandes catégories de tailles, une situation possible pour certaines pêches à la ligne. Nous présentons des exemples de sélection pour des pêches à la ligne, au filet traînant et aux hameçons mixtes, auxquelles la courbe logistique à pic se prête mieux que les courbes de sélection lognormales et binormales comparatives. Nous mettons également en avant une autre solution, à savoir la courbe normale bilatérale. [Traduit par la Rédaction]

CPUE comparison of traditional crayfish fyke net rigged with knotted net and alternatively knotless net

“…Generally, nets with thin ropes are considered more productive compared to the ones with thick ropes as they are less visible, easier to stretch and more flexible (Grati et al, 2015;Hamley, 1975, Jensen, 1995Turunen, 1996); accordingly, the findings of our study are similar to those of other studies. Our study has demonstrated that the same situation applies to the fyke nets used in crayfish fishing.…”

Fish attraction to artificial reefs not always harmful: a simulation study

“…We defined attraction as a function of distance between the natural and artificial reefs (closer reefs attract more fish) and of relative reef quality (higher quality artificial reefs will attract more fish). A logistic function (Smith and Taylor ) was used to define the amount of fish biomass in a given cell that is attracted to the artificial reef:where is the relative biomass remaining in cell i after attraction, Q 0 is a reef quality parameter, D 99 is the distance over which almost all (99%) attraction occurs, and D 50 is the distance at which half the fish in a cell will be attracted a reef. D 99 was fixed at twice the value of D 50 to ensure a symmetric curve.…”

“…We defined attraction as a function of distance between the natural and artificial reefs (closer reefs attract more fish) and of relative reef quality (higher quality artificial reefs will attract more fish). A logistic function (Smith and Taylor 2014) was used to define the amount of fish biomass in a given cell that is attracted to the artificial reef:…”

Technical and Design Characteristics of Trammel Nets Used in Lake Manzalah, Egypt

“…The monofilament net twine is more elastic and flexible than multifilament. Meshes of a more elastic twine can be stretched to be larger size by a struggling fish (Hamley, 1975). However, there was no systematic relation among the sizes of fishes captured in the same mesh-sized nets of the net types.…”

“…Horizontal hanging ratio (E) is defined as the ratio between the stretched head rope or lead rope relative to the total stretched netting mounted on that rope and it gives the distinctive shape of the mesh. It was shown that the hanging ratio is amongst many factors affecting the selectivity of fishes (Clarke, 1960 andHamley, 1975). However, the effects in selectivity of changing the hanging ratio are less studied than those of mesh size.…”