ANNEX 10 



Definitions and Methods of Measuring and Counting 



in the Billfishes (Istiophoridae, Xiphiidae) 1 '' 



by 



Luis Rene Rivas 



Abstract 



The need for definition and standardization of methods of measuring and 

 counting in ichthyology is discussed, with special reference to billfishes. A 

 series of measurements and counts for the latter group is proposed and 

 methods and definitions for each are given. The body length is discussed in 

 more detail in connection with its importance as a base length and attention 

 is called to the need for dissection in order to ascertain accurately the number 

 of spines in the first dorsal and first anal fins. 



Introduction 



The need for accurate definition and standardization in the use of 

 biometric and meristic characters in systematic ichthyology has long been 

 recognized (Ricker and Merriman, 1945). It is obvious that with the exception 

 of truly self-explanatory characters, most measurements and counts must be 

 defined in order to enable other workers to interpret the data. The need for 

 standardization arises from the fact that in most cases, different methods (as 

 applied to a given group), no matter how well defined, cannot be equalized 

 for comparative purposes. 



Owing to the high precision required, lack of definition and standardiza- 

 tion of methods becomes quite a problem in the study of closely related 

 species or infraspecific categories, and especially in the biometric analysis of 

 populations where several independent workers using different methods may 

 be working on the same group. Furthermore, the marked differences in struc- 

 ture existing among certain families of fishes usually prevent the application 

 of a method to groups other than the one for which it was designed. 



In recent years, the increasing interest in the biometric analysis of pop- 

 ulations of tunas by various independent workers, has brought about the 

 necessity to define and standardize the methods used in measuring and count- 

 ing. The various methods which have been proposed are essentially in agree- 

 ment (Godsil and Byers, 1944:125-129; Marr and Schaefer, 1949; Rivas, 1955) 

 and have been successfully adopted by practically all workers in the field. 



Also recently, new interest has developed in the taxonomy and population 

 analysis of the sailfishes, spearfishes, marlins (Istiophoridae) and broadbill 

 swordfish (Xiphiidae), a most confused group collectively known as 

 "billfishes." 



As far as can be ascertained, no formal methods of measuring and counting 

 have ever been proposed for the billfishes. A survey of the literature shows 

 that most of the methods used vary among the different workers and that 

 lack of definitions renders the measurements and counts difficult or impos- 

 sible to interpret. In addition, certain methods of measuring and counting 

 employed in the past appear to be unsatisfactory and have resulted in 

 questionable taxonomic interpretations. 



For reasons already indicated above, the methods employed in the tunas 

 cannot be applied to the billfishes. It is the purpose of the present paper to 

 propose a series of measurements and counts for the latter group, based on 

 previous field and laboratory experience as a result of studies conducted un- 

 der sponsorship of the Charles F. Johnson Foundation. New characters not 

 previously used in connection with billfishes are also included. 



All the measurements described (excepting body girth) are straight-line 

 distances and are made in metric units to the nearest millimeter, with slide 

 calipers or dividers according to the size of the fish and the distance to be 

 measured. (See also Godsil and Byers, 1944:125, and Marr and Schaefer, 

 1949:241, 242). In large fish, long measurements beyond the range encom- 

 passed by the larger calipers may be made with a steel tape graduated in 

 metric units. For this purpose sliding metal or wooden arms similar to those 

 used in the calipers should be attached to the tape, taking care that the tape 

 remains straight during the measurement, with the arms perpendicular to it. 

 As already pointed out by Morrow (1952:53, 54), measurements taken with a 

 tape alone are not satisfactory, since a straight line distance can seldom be 

 obtained. 



Also, in order to avoid error in the longitudinal measurements, the axis of 

 the body should be maintained as straight as possible. This may be ac- 

 complished by placing the specimen on a flat surface and properly propping 

 up the head, the caudal peduncle and the caudal fin. Although it is conven- 

 tional in ichthyology to use the left side of the fish for the lateral 



'Contribution No. 149 from the Marine Laboratory, University of Miami. This consti- 

 tutes a technical report to the Charles F. Johnson Foundation 



'From Bulletin of Marine Science of the Gulf and Caribbean 6(l):18-27, 1956. Reprinted 

 with permission of editor. 



measurements, the best, or either side should be selected, according to the 

 condition of the part to be measured. The jaws should be tied closed, es- 

 pecially in connection with measurements involving the tip of the mandible 

 as a point of reference. 



The numbers in the text for each measurement correspond to the numbers 

 in the figure. 



Measurements 



1.— Body length.— A survey of the literature shows a great deal of 

 inconsistency as to the selection of a body length in billfishes and lack of 

 definition whereby the points of reference of this measurement can be ac- 

 curately established. With very few exceptions, the instrument employed is 

 not mentioned and there is no statement as to whether or not the measure- 

 ment follows the curvature of the body (tape) or constitutes a straight-line 

 distance (calipers or dividers). 



It must be emphasized that since most, or all, other (relative) body 

 measurements are referred to the body length as a base length, regardless of 

 the method used in expressing proportions (ratios, regressions, etc.), this 

 character must be defined with great care. It is obvious that if the base length 

 is in error, all body proportions will also be in error regardless of how ac- 

 curately the body parts may have been measured. 



The "standard length" or "body length" for billfishes as used by most 

 workers in the past does not seem to be satisfactory for various reasons. There 

 has been agreement in the selection of the anterior end-point (tip of bill) but 

 the posterior end point is variously interpreted as " . . . tail base" or " . . . 

 mid-point of the peduncle . . ." (Conrad and LaMonte, 1937, table 1 and p. 

 209); or " . . . the midpoint of the shallowest vertical diameter of the caudal 

 peduncle," (Gregory and Conrad. 1939:444), etc. Other workers offer no 

 definitions or simply refer to "standard length" (deBuen. 1950:171) without 

 further comment. 



Despite lack of absolute standardization (Ricker and Merriman, 1945), 

 most ichthyologists agree in that "standard length" is the straight line 

 measurement taken between the tip of the snout and the middle of the caudal 

 base, where the middle caudal ray joins the last (hypural) vertebra. In the 

 billfishes. however, the middle of the caudal base cannot be determined 

 without involved dissection, and the structure of the hypural vertebra and 

 the caudal fin do not permit the determination of an accurate point of 

 reference. Even after performing dissection, the point cannot be estimated 

 from external form. For obvious reasons, the middle point on the posterior 

 margin of the middle caudal rays (crotch of tail) constitutes a much better 

 point of reference from the point of view of accuracy and convenience. In ad- 

 dition, the median caudal rays in billfishes are well protected by the upper 

 and lower lobes of the fin, and are very seldom damaged. 



As to the anterior point of reference, the tip, or a considerable portion of the 

 distal end of the bill is frequently broken off, or the bill itself may be 

 malformed and not attain its true length. For this reason, many otherwise 

 valuable specimens have to be discarded or an inaccurate body length will 

 result if the tip of the bill is used for the anterior point of reference. The man- 

 dible, on the other hand, is well protected by the bill and its tip is very seldom 

 broken off or malformed. 



In the light of the above discussion, it is therefore proposed that the body 

 length in billfishes be measured between the tip of the mandible (with the 

 jaws closed) and the middle point on the posterior margin of the middle 

 caudal rays. 



2. — Body girth. —Measured with a tape on one side of the body following its 

 curvature from the uppermost point on the edge of the dorsal groove, vertical- 

 ly to the edge of the pelvic groove (midline of belly in the swordfish); the 

 resulting figure is then multiplied by two. This character, when expressed as 

 a proportion of the base length, serves as a good indicator of the degree of 

 robustness of the body. 



3.— First predorsal length— Measured from the tip of the mandible to the 

 origin of the first dorsal fin. The latter point is the intersection of the anterior 

 margin of the fin with the contour of the back when the fin is held erect. 



4.— Second predorsal length.— Measured from the tip of the mandible to 

 the origin (as defined above) of the second dorsal fin. The origin of the second 

 dorsal is not as clearly defined as that of the first, and the point must be es- 

 timated as accurately as possible. Since this is a long measurement, the 

 error, if any, is negligible. 



b.—Prepectoral length.— Measured from the tip of the mandible to the 

 origin of the pectoral f in . The origin of the pectoral fin is the intersection of its 

 anterior basal margin with the side of the body, when the fin is held erect. 



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