Growth Pattern And Cannibalism In Black Rockfish (Sebastes Schlegelii) During Early Development Stage

Black rockfish (Sebastes schlegelii) is a demersal economic species, widelydistributed in the coastal regions of China. The quantity of black rockfish resource hasdecreased significantly in recent years because of environmental and human factors.Artificial culture and stock enhancement become significant methods of commercialproduction and resource complement. To know more about the rule of earlydevelopment of black rockfish and provide reasonable strategies of enhancement andcultivation, this paper has studied on the allometric growth and the development ofvisual system in black rockfish during early stage, and several proposals are presentedto predict, prevent and control the high frequency of cannibalism in breeding period.Larvae, juveniles and young fish of S. schlegelii, aged from0to90d, weremeasured for morphological indexes (such as total length, body weight, head length,head depth, snout length, eye orbit diameter, body depth, fin ray length, fin spinelength, etc.). Then these indexes were regressed on total length to confirm theirallometric growth patterns and occurrence time of inflection points, which wereassociated with the ecological habits of larval and juvenile to discuss the growthpattern and ecological significance of black rockfish during early development stage.The first inflection point was found in the caudal fin rays. The caudal fin ray emergedon6day age and grew extremely rapidly (growth coefficient b=4.37) until12dayage (inflection point), then it grew normally afterwards (b=1.08). And this allometricgrowth pattern helped the larval acquire more comprehensive swimming ability,which was significant to feeding and avoidance. Meanwhile, the regression totallength and body weight on day age were both cubic curves. The regression of totallength on day age，LT=-0.0001D3+0.015D2-0.0162D+6.0304(R2=0.98), couldbe divided into three phases (0~16d、17~46d、47~90d), with significant difference in growth velocities. And the highest growth velocities appeared in thethird phase, which entered into young stage. However, the regression of body weighton day age, WB=-3E-06D3+0.0008D2-0.0213D+0.1052(R2=0.98), could bedivided into two phases (0~46d、47~90d), with significant difference in growthvelocities.Aiming to confirm the function of visual system in the early development, theretinae of S. schlegelii in0~90d were conducted to histological observation. Theretina had10layers and the eye ball was well developed when the larval born, owingto ovoviviparity. The development of visual system could be divided into three phases:0~24d, during this period, the thickness of every layer increased rapidly. Thephotoreceptor cells were single cones and no rod cell had been observed. Cells inouter nuclear layer were bar-type. The quantity of ganglion cells changed not muchbut every cell became bigger, and the degree of the assembled meshwork is increased.Also, the minimum separable angle decreased rapidly therefore the visual acuityincreased.24~50d, the phase of greatest change in visual system, the outer nuclearlayer became granule status, and rod cells and double cones emerged. Meanwhile,single cones and double cones generated the mosaic shape, which improved the visualsense of movement. During this period, the ecological habit of black rockfish shiftedto demersal layer from pelagic layer.50d afterwards, the retina changed not muchtherefore the visual system was deemed to be well developed.In the research of early development, every inflection point of eachmorphological indexes and the dividing points of visual system development stagesgathered in the points of metamorphosis of black rockfish from larval to juvenile (14~24d) and juvenile to young fish (46~60d), which confirms the changes onmorphological feature of larva and juvenile organs synchronize with metamorphosis.To confirm the critical condition of cannibalism occurred in black rockfish, themouth gap and body depth were regressed on total length respectively, which couldlimit occurrence of cannibalism in morphology. The regressions were: GM=0.166LT+0.258(R2=0.98)，DB=0.266LT+0.036(R2=0.99), respectively. Based on them,the critical condition of cannibalism (i.e. theoretical model) was presumed: LT prey= 0.624LT cannibal+0.835. To verify or revise this model, black rockfish from20to72mm total length were paired to observe, and the revised model was obtained: LT prey=0.522LT cannibal+4.908(R2=0.99), which could be used to grade black rockfish bysize to remove potential cannibals.