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240 E E Z ib v C 4) O 0 ii 3 160 140 120 100 80 00 40 20 ? P. L1x lw jaw gape a X. texanus dommentral depth y = 0.196. - 2.517 • C. tatipinnis dorsoventral depth a R'=0.984 a ° y = 0.1415x - 0.729 8 ° R'=0.985 ? r • y=0.0615.+1.769 .w R' = 0.911 0 200 400 600 800 1000 Total length (mm) Figure J. Jaw gape (dorsoventral dimension) of P. Lucius and dorsoventral depth at nuchal region of X. texanus and C. Ian- pinnis, as a function of total length. Individual fishes are rep- resented as P. lucius (black diamonds), X. texanus (squares), and C. latohmis (gray circles). E E n m M c 0 12 00 CL rn 160 140 ' P. Ludus law gape y = 1776x -1.949 0 G. cypha dorsoventral depth R' = 0.996 120 •O. robusts dasoventral depth 100 y = 0.149% + 0.454 R' = 0.987 80 ° o d) 60- 40- ? v=o.o6lsx+t769 ? R'=0 911 20 . 0- 200 400 600 800 1000 Total length (mm) Figure 6. Jaw gape (dorsoventral dimension) of P. Lucius and dorsoventral depth at nuchal region of G. cypher and G. robusta, as a function of total length. Individual fishes are represented as P. lucius (black diamonds), G. cypha (squares), and G. robusta (gray circles). that P. lucius piscivory was limited to only the smaller sizes of fishes with enlarged humps (Fig- ures 5 and 6). In addition, it appears that nuchal humps provide protection from P. lucius predation early in development (i.e., for small total length). For example, according to the data presented in Figures 5 and 6, about 73% of the total length range for C. latipinnis and 83% for that of G. ro- busta could be consumed by the largest P. lucius (i.e., 805 mm), with only the very large fish gaining protection, whereas, only 55% of the total length range for X. texanus and 71% for that of G. cypha could be ingested by the largest P. lucius (Figures 5 and 6). Discussion A fair evaluation of the `nuchal hump hydrody- namic advantage theory' required laboratory testing with the fish body casts, and the results were not surprising. Just as the wings of an air- plane generate lift, a prominent hump on a fish also results in lift, which would make holding its position on the bottom more difficult. In addition, increased drag arises from frictional and pressure forces associated with a hump. Frictional forces for a given body can be reduced by maintaining laminar boundary layer conditions over as much of the body as possible. At moderate and high Reynolds numbers, any object from which flow separates will experience a relatively high drag. If the object is streamlined, fluid gradually deceler- ates in the rear with little separation occurring and the object is actually pushed forward by the wedge-like closure of the fluid behind it. Efficiency of a moving streamlined body through water de- pends on the positioning of the area of the body with the largest girth (shoulder). Fast-moving fishes, such as scombroids, have their shoulder situated far back on the body, which extends the influence of the favorable pressure gradient and encourages laminar boundary layer flow over most of the body (Webb 1975, Vogel 1994). A stream- lined body usually has its maximum thickness at least 30% from its front (Denny 1988, Vogel 1994, Diana 1995). However, G. cypha and X. texanus have bodies in which the transitional point be- tween laminar and turbulent flow is practically at the leading edge. The nuchal hump causes the shoulder to be very much anterior, which results in the immediate loss of a laminar boundary layer and an unfavorable pressure gradient. Therefore, these nuchal humps are a hindrance to the swim- ming ability of G. cypha and X. texanus. We investigated whether enlarged nuchal humps might convey a hydrodynamic advantage for life in fast flowing water, and especially for facilitating position holding in stream flows. Thus, we were not concerned with measuring thrust power or complex fluid mechanics of these fishes, but rather