age and growth of pontinus kuhlii (bowdich, 1825) in the canary

AGE AND GROWTH OF PONTINUS KUHLII 259

INTRODUCTION

The Scorpaenidae Pontinus kuhlii (Bowdich,1825), commonly called offshore rockfish, is a dem-ersal species that inhabits the rocky substratum ofslope at depths that range from 100 to 460 metres(Nunes, 1974; Eschmeyer and Dempster, 1990;Franquet and Brito, 1995; Menezes, 1996). Theirgeographical distribution extends from the AzoresArchipelago and the coast of Portugal to Angola;although their presence in the Mediterranean Seaand South African waters is uncommon (Hureau andLitvinenko, 1986; Eschmeyer and Dempster, 1990;Merella et al., 1998).

In the Canary Islands and surrounding banks, thisspecies, although of commercial value, is part of theby-catch of the artisanal and industrial longlinerswhose target species is another Scorpaenidae, thebluemouth Helicolenus dactylopterus (Delaroche,1809), which is a more abundant species than P.kuhlii in this region. These two species partiallyoverlap their bathymetric distribution between 200and 400 metres in the area surrounding the CanarianArchipelago, P. kuhlii inhabits the shallower waters.

Knowledge of the biology of this species and lit-erature on the subject are very scarce. Isidro (1990,1996) has contributed with data on growth andreproduction, as well as a preliminary study on man-agement of the stock in the waters of the Azores.Krug et al. (1998) provided a summary of growth

SCI. MAR., 65 (4): 259-267 SCIENTIA MARINA 2001

Age and growth of Pontinus kuhlii (Bowdich, 1825) inthe Canary Islands*

L.J. LÓPEZ ABELLÁN, M.T.G. SANTAMARÍA and P. CONESA

Centro Oceanográfico de Canarias, Instituto Español de Oceanografía, Carretera de San Andrés, s/n, 38120 Santa Cruz de Tenerife. España. E-mail: [email protected]

SUMMARY: Pontinus kuhlii is a Scorpaenidae which forms part of the bottom longline by-catch in the Canary Islands fish-eries within setting operations from 200 to 400 metres depth. Information on their biology and on the age determination andgrowth of the species is very scarce. The main objective of the study was to look at this biological aspect based on 421 spec-imens caught in the Canarian Archipelago during the period July 1996 and August 1997, 286 of which were male, 130female and 5 indeterminate. Age was determined through the interpretation of annual growth rings of otoliths and scales,and the results fitted the von Bertalanffy growth function. Otolith sectionings were discarded due to annuli losses caused bythe edge structure. The structures used in age interpretation showed numerous false rings that made the process difficult,causing 17% of the cases to be rejected. However, the age interpretations from scales showed less variability in relation tothe whole otolith. The ages of the specimens ranged from 6 to 18 years for males and from 6 to 14 years for females. Thegrowth parameters for males were: K= 0.132, L∞ = 46.7 and to= 1.74; for females: K= 0.094, L∞= 46.3 and to= 0.05; and forthe total: K= 0.095, L∞= 52.2 and to= 1.01.

Key words: age, growth, Pontinus kuhlii, Scorpaenidae, Canary Islands, Eastern Central Atlantic.

*Received May 12, 2000. Accepted February 2, 2001.

estimations within the same geographical area.Other authors have concentrated mainly on descrip-tions of new sites or revising their area of distribu-tion. The present work aims to improve knowledgeof age determination and growth of this species, andwas carried out within the framework of the DG-XIV project (95/032), focused on the biology ofdeep-sea species covering the Macaronesian archi-pelagos of The Canaries, Madeira and The Azores.

Our objective includes the study of the age andgrowth of P. kuhlii from the Canary Islands Archi-pelago and adjacent waters. It describes the processof reading and interpreting age. Growth parametersare estimated and compared with the availableresults on this species (Isidro, 1996; Krug et al.,1998).

MATERIAL AND METHODS

The samples used came from commercial land-ings obtained in ports of the Canarian Archipelagobetween July 1996 and August 1997 (Table 1), fromships which habitually work in these waters and sur-rounding areas (Fig. 1). A total of 421 individualswere used, of those 286 were male with a size rangeof between 17.8 and 46.6 cm in total length, 130were female with lengths of 17.7 to 38.4 cm and 5indeterminate (15.7-23.2 cm). In addition to the totallength (mm) measuring and sex determination,otoliths were extracted from each individual andstored without any previous treatment. Cleanedscales were prepared between two slides to be storedfor later observation and age determination.

260 L.J. LÓPEZ-ABELLÁN et al.

TABLE 1. – Sampling for biological analysis of P. kuhlii. Canary Islands. Depth range: 200-400 m.

Date Island n Size range Date Island n Size range(month/year) Total Length (cm) (month/year) Total Length (cm)

07/96 La Graciosa 13 26.1-43.7 02/97 Lanzarote 7 30.4-39.909/96 Lanzarote 38 24.3-42.7 03/97 Lanzarote 14 29.5-43.010/96 Lanzarote 19 21.2-41.4 03/97 Gran Canaria 14 24.9-42.010/96 Tenerife 1 27.2 03/97 Tenerife 18 19.6-37.211/96 Lanzarote 32 21.0-46.6 04/97 Fuerteventura 82 15.7-41.212/96 Lanzarote 27 26.1-43.8 05/97 Fuerteventura 10 17.8-46.101/97 Lanzarote 9 22.8-41.8 05/97 Lanzarote 16 19.8-36.001/97 Gran Canaria 2 37.0 06/97 Tenerife 18 22.0-40.502/97 Gran Canaria 3 18.0-23.2 08/97 Fuerteventura 50 20.6-42.502/97 Fuerteventura 48 22.4-44.1

20°� 15°� 10 °�

Longitude

Lat

itude

N

W

25°�

30°�

°�35

WESTERNSAHARA

Canary Islands

Madeira

ATLANTICOCEAN

�

FIG. 1. – Studied species (Pontinus kuhlii (Bowdich, 1825)) and area where the specimens were caught.

Age was estimated by means of the whole sagit-tal otoliths (Fig. 2A) immersed in water using astereo microscope with reflected light, and the directreading of growth rings on scales (Fig. 3) using aprofile projector. Trials with sectioned otolithsshowed losses of annuli for individuals older than10/11 years due to the great increase of crests in thesagitta edge with regard to whole otoliths in whichgrowth rings can be identified on the crests.

The date of birth assumed was January 1st. Thelack of young fish did not allow us to assess the evo-lution of the marginal ring in our study. However,the nature of rings were considered as annual (onehyaline and one opaque ring constitute an annualgrowth zone), regarding age validation on thisspecies in the Azores (Isidro, 1996).

Readings of both structures were carried out bytwo people independently, and age for each speci-men was adopted depending on the reliability ofreading of both structures. When discrepanciesoccurred , a third reader intervened trying to corrob-orate previous observations; if this couldn’t be done,the readings were rejected. The variations “between-readers” were analysed using the number of ringsassigned to each structure and the differencebetween them. To determine the suitability of themethod, the structures used and the precisionachieved, a comparative study of the reading results

from scales and otoliths of two first readers was car-ried out using the method by Eltink (1994). At thesame time, the dispersion of individual agesassigned from scales and otoliths was analysed.

The age-length relationship was establishedthrough the corresponding keys that were elaboratedin number of individuals for males and females, adopt-ing size intervals of one centimetre and rounding off tothe lower centimetre. In addition, the mean size andthe standard deviation by age class was estimated tak-ing as a reference the middle point of the interval.

The number of males, females and total of indi-viduals in the landings was estimated at one cen-timetres intervals. In addition, the age-length keyswere applied in order to obtain the number and per-centage of individuals by sex and for the total land-ing, by age class.

The growth parameters for males, females and forall individuals were estimated using the least squaremethod, fitting the length and age values to the vonBertalanffy growth function (Lt = L∞ [1-e –K(t-to)])(Tomlinson and Abramson, 1961). Indeterminateindividuals were assigned to males and females at50%. Finally, a covariance analysis (Zar, 1984) wasapplied in order to determine if there were statistical-ly significant differences between length and age val-ues used to obtain the male and female growth curves,to detect differences in growth by sex.

RESULTS

Age determination

Both otoliths and scales showed many false rings(Fig. 3). The otoliths also presented numerous ram-


FIG. 2. – (A) Pontinus kuhlii otolith with 8 years assigned. The read-ing line shows the most common path used to identify annual rings.

(B) Pontinus kuhlii otolith showing (arrows) edge ramifications.

FIG. 3. – Pontinus kuhlii scale with 13 years assigned. The readingline shows one of the possible paths to follow to identify annual rings.

ifications at the edges, adding some complexity tothe process of reading (Fig. 2B). This otolith struc-ture is common for individuals of 10/11-yrs, and theramifications are more complex as individuals getolder.

Generally, in both otoliths and scales, it wasfound that the ring considered as the third ring wasalways very visible or more easily recognised thanwere the first and second. On the other hand, thescale rings from the sixth or seventh to the edge arethose that were identified more clearly.

Comparing the results of the first two readers inthe initial phase, it was observed that they coincidedin 52% of cases. After the third reader’s interven-tion, the age assignments increased by 31%, and17% had to be rejected.

Analysing the variations “between-readers”,coincidences between otolith and scales readingswere 46.5% and 30.3% respectively, with frequentdifferences of ±1 and ±2 years. There was a tenden-cy toward the underestimation in ages determinedfrom otoliths in relation to ages from scales. This isalso found when comparing the ages assigned byboth readers to the otoliths and scales of all individ-

uals (Fig. 4). However, no inflection takes placewithin the cloud of points that could indicate a dif-ferential approach in the assignment of age in scalesand otoliths associated with the increase of analysedages. This only happens at ages 15 and 18 and couldbe due to the scarce number of individuals analysedfor these ages (12 and 2 respectively).

The comparative study of readings by structureand reader shows that the maximum precision wasobtained in the scale readings (12-15% CV) whileless precision (14-17% CV) was obtained in thewhole otolith readings (Table 2). Variability of read-ings in relation to the adopted final age increasesfrom age 10-yr (Fig. 5), but, in general the meanvalue is very close to the final age. The bias of thismean value, in relation to the adopted age, was notmore than 1 year of difference with the exception ofthree ages. At ages less than 10-yrs the bias was lessthan 1 year.

Age-length relationship

The ages of the studied individuals rangedbetween 6 and 18-yrs, coinciding with the range ofages of the males. The females reached ages between6 and 14-yrs and the indeterminate ones between 6,7 and 8-yrs. Using age-length keys corresponding tomales and females (Tables 3 and 4 respectively), thebest represented age classes were included between 7and 12-yrs. Also, the mean size for each age class ofthe total evolves appropriately until reaching age


TABLE 2. – CV (%) of readings and weighted mean of CV

Adopted Age 6 7 8 9 10 11 12 13 14 15 16 17 18 6-18

Reading 1 (Otolith-Reader 1) 18% 20% 20% 11% 17% 17% 17% 14% 11% 18% 10% 19% 34% 17%Reading 2 (Scale-Reader 1) 18% 14% 16% 11% 11% 14% 10% 13% 11% 10% 5% 4% 4% 12%Reading 3 (Otolith-Reader 2) 10% 13% 14% 19% 14% 10% 17% 18% 17% 8% 8% 0% 14% 14%Reading 4 (Scale-Reader 2) 16% 20% 16% 17% 14% 14% 16% 12% 16% 9% 6% 11% 4% 15%

FIG. 4. – Individual age determined from scales and from otoliths. Line is scale age = otolith age.

FIG. 5. – Age bias plots. The mean age recorded +/- 2stdev of all agereadings (otolith and scales) is plotted against the adopted age.

classes 13 and 14, in those an increment that could beconsidered anomalous takes place.

Size and age composition of catches

In Figure 6 the size distributions of catches (thatcoincides with the analysed individuals) are present-ed for males, females and indeterminates. The lowerlimit in both for males and for females was 17 cm.However, males reached a maximum much largerthan that of the females (46.0 cm and 38.0 cmrespectively).

The transformation of these size distributionsinto ages (Table 5) shows that males are better rep-resented at age classes 10, 11 and 12-yrs (13%-18%), followed by 9-yr (11.5%) and 14-yrs (10.4%).As for females the ages better represented wereequally the 10-yr (22%), 8-yrs (19.5%) and 9-yrs(17%). In general, considering the total of individu-als, the majority of individuals caught and analysedreached ages included between 9 and 12 years, and19% of the total were 10-yrs.

Growth

The parameters that define the growth of P. kuh-lii by sex and for the total of individuals as well asthe number of individuals used and the range ofsizes are presented in Table 6. The annual K esti-


TABLE 3. – Age-length key, mean length and standard deviation. Number of males of each age class

Length / Age 6 7 8 9 10 11 12 13 14 15 16 17 18(cm) (yr)

151617 118 119 120 3 1 1 221 1 2 122 1 2 123 3 2 3 1 124 2 1 1 225 1 1 2 1 126 1 1 2 1 127 2 3 3 328 1 1 6 7 429 1 2 2 3 230 1 4 4 2 131 1 5 2 3 232 3 4 3 2 1 2 133 1 1 1 2 1 134 2 4 3 335 3 4 1 2 136 1 2 1 1 1 2 137 1 1 2 3 4 1 3 138 2 4 1 2 1 139 2 1 1 3 1 1 140 1 2 2 5 141 1 3 1 2 3 242 1 1 1 2 2 143 2 1 1 144 1 14546 1

Mean length (cm) 20.30 23.79 25.25 29.11 31.64 32.12 34.09 37.44 39.21 37.58 38.90 40.00 41.00(SD, cm) 1.8 3.9 3.7 4.8 4.8 4.6 5.6 4.6 3.0 3.8 2.3 3.0 2.1N 5 14 12 28 44 37 32 16 24 12 10 6 2

FIG. 6. – Length frequency distribution by sex of the total individu-als analysed.

mated varied between 0.094 and 0.132 and the L∞between 46.3 and 52.2 cm. The growth curvesincluding the size ranges, the mean values of theobservations and their confidence limits, for males,females and the total of individuals, are presented infigures 7, 8 and 9 respectively. Males show a supe-rior K to that of the females but the L∞ is very simi-lar. The growth parameters of the combined curvewere: L∞ (52.2 cm, 6.7 SE) and the K (0.095 cm,0.031 SE).

Lengths corresponding to the fitted curves do notmove away from mean lengths of the individuals for


TABLE 4. – Age-length key, mean length and standard deviation. Number of females of each age class

Length / Age 6 7 8 9 10 11 12 13 14 15 16 17 18(cm) (yr)

15 11617 118 2 119 120 121 3 122 2 4 4 323 1 3 1 124 1 1 325 1 3 2 4 1 126 1 1 4 2 127 3 4 228 1 1 229 1 1 6 1 130 1 1 3 131 1 1 1 132 4 1 133 1 1 134 1 1 535 13637 138 13940414243444546

Mean length (cm)16.50 22.41 25.31 26.85 26.96 30.65 31.58 31.50 35.50(SD, cm) 1.4 2.5 3.2 3.7 3.4 3.6 4.2 - 4.2N 2 11 21 20 26 13 12 1 2

TABLE 5. – Age composition of catches of P. kuhlii by sex and total

Age class 6 7 8 9 10 11 12 13 14 15 16 17 18

Males 5 17 13 33 52 44 37 20 30 14 13 8 2% 1.7 5.9 4.5 11.5 18.1 15.3 12.8 6.9 10.4 4.9 4.5 2.8 0.7Females 2 14 26 23 29 20 14 2 3% 1.5 10.5 19.5 17.3 21.8 15.0 10.5 1.5 2.3 - - - -Total 7 31 39 56 81 64 51 22 33 14 13 8 2% 1.7 7.4 9.3 13.3 19.2 15.2 12.1 5.2 7.8 3.3 3.1 1.9 0.5

FIG. 7. – Growth curve of Pontinus kuhlii males. Vertical lines represent length range observed and bars indicate mean +/- 2SE.

age class that evolve with gradual increments untilage 13-yrs, when some fluctuations take place aboveand below the growth curve. The standard error ofmean sizes decreases considerably when consideringall the data as a whole (Fig. 9) and consequently theconfidence limits are affected in the same way.

The covariance analysis showed that there arestatistically significant differences (P <0,05)between the size-age relationship of females andthat of males.

DISCUSSION

The difficulties found in the current work in rela-tion to the age interpretation of P. kuhlii were alsoshown by Isidro (1996) working with otoliths of thisspecies caught in the Azores, establishing that 36%of these were difficult to read or illegible. In theCanaries that difficulty would be located at around48%, with the interpretation problems growing fromthe age of 10-yr.

The tendency to underestimate age from otolithscould be due to a systematic error in the interpreta-

tion of rings/false-rings in each of the structures; tothe presence of irregular edges in otoliths startingfrom a certain age that hinders their interpretation;or probably to the greater ease with which growthrings in the scales are detected, mainly from thesixth or seventh ring to the edge.

Regarding the results obtained, just as otherauthors found for this and other species of the samefamily, and despite the complex reading process ofboth structures, everything seems to indicate thatthese are valid for the determination of age in thecurrent case, which is not so in other species whosescales are rejected (Bergstad, 1990). Beamish andMcfarlane (1987) do not recommend the use ofscales in age determination because of the underes-timation of ages of older fish. However, in the caseof P. kuhlii, scales not only provide valid comple-mentary information for the assignment of age, butin the case that it were necessary to choose only asingle structure to determine age, there is no reasonit should not be the scale. Likewise, the use of wholeotoliths in spite of difficulties with readings, couldbe justified taking into account that sectionedotoliths lose annuli in the major axis because of the


TABLE 6. – Estimated parameters of von Bertalanffy growth equation of P. kuhlii in the current and other studies, size range and number ofspecimens (O: otolith reading; S: scale reading; M: males; F: females; T: total; n: number of individuals; TL: total length; L∞: asymptotic

length, K: growth coefficient, to: theoretical age when length is zero).

Source/Area Method Sex n Size range TL (cm) L∞ (cm) K (yr-1) t0

Current study O+S M 242 17.80 - 46.60 46.7 0.132 1.74(Canary Islands) O+S F 108 15.70 - 38.40 46.3 0.094 0.05

O+S T 350 15.70 - 46.60 52.2 0.095 1.01Isidro (1996) O M 367 14.50 - 49.50 54.2 0.076 -1.96(Azores) Back-calculation 55.8 0.092 -5.58

O F 269 14.50 - 40.50 38.2 0.120 -1.73Back-calculation 46.0 0.115 -5.72

O T 662 13.50 - 49.50 51.7 0.076 -2.10Back-calculation 54.4 0.093 -5.58

Krug et al. (1998) O M 402 - 80.2 0.037 -3.82(Azores) O F 396 - 43.3 0.096 -1.98

O T 824 - 73.6 0.039 -3.87MULTIFAN T 1171 - 51.4 0.106 -0.55

FIG. 8. – Growth curve of Pontinus kuhlii females. Vertical lines represent length range observed and bars indicate mean +/- 2SE.

FIG. 9. – Growth curve of Pontinus kuhlii for males and females.Vertical lines represent length range observed and bars indicate

mean +/- 2SE.

edge ramifications and growth rings in minor axisare difficult to identify at the outer margin. The useof whole otoliths in age determination is closelyrelated to species, as could be concluded from thestudy done by Maceina and Betsill (1987), althoughlongevity makes this more difficult.

Considering both structures for age determina-tion in this species, the analysis of the bias in theobservations has shown an increase in the variabili-ty of the mean values in relation to the final ageadopted as early as 10-yrs. This could be due to thedifficulty in the interpretation of age from otolithbecause of the ramification of their edges. Also, thepositive difference in most cases between the meanand the adopted age could be due to the greater num-ber of rings detected in the scales in relation to theotoliths. This means that, the adopted final ageapart, the value of the average of the readings tendsto be the same or superior to it. Whichever the case,the scarce difference between the average of thereadings and the adopted final age (less than 1 yearin most cases), shows the coherent system of ageinterpretation applied by the reading team consider-ing the results obtained.

Isidro (1996) reported for P. kuhlii an age rangeof 2/18-yrs corresponding to 13.5/47.5 cm of totallength, which is very similar to ages 6/18-yrs corre-sponding to 15.7/46.6 cm of total length found inthis study. Thus, results on age determination andthose related to the different fittings carried outseem to be within limits that can be consideredacceptable and they indicate that P. kuhlii is a fishwith slow growth. However, in all cases and espe-cially in that of the females, principally due to hookselectivity and accessibility to the resource, a short-age of individuals in the 15-yrs age class and anabsence of young specimens in ages below 6-yrs isobserved. Consequently, the theoretical growthcurve lacks size-age pairs of values of reference inthe initial and end sections. Considering these fac-tors, and in spite of indications or evidence thatthere is differential growth between sexes anduncertainties associated to the fitting carried out bysex, the use of the parameters of the total equation isadvisable.

The results obtained for all individuals are ofthe same order as the estimates made by Isidro(1996) for P. kuhlii in Azores waters (Table 6),mainly from values of back-calculation (K= 0.095,0.093 and L∞ = 52.2, 54.4, respectively). The esti-mations of Krug et al. (1998) show a great vari-ability, although there is some agreement with the

current study for females (otolith reading) and forthe total (MULTIFAN).

Comparing these results with those of otherspecies of the Scorpaenidae family like the previous-ly mentioned Helicolenus dactylopterus, which over-laps in habitat with the target species of this study, itis observed in Kelly et al. (1999) that although theyreach an inferior asymptotic length (males 37.2 cm,females 31 cm), the growth constants (0.06 - 0.09) aresimilar to those estimated for P. kuhlii in the currentwork. Similar values for the growth constants(between 0.09 and 0.18) were found by Isidro (1987),Ragonese and Reale (1992), D’Onghia et al. (1996),Esteves (1997), Krug et al. (1998) and Massuti et al.(2000). White et al. (1998) also found, for thisspecies, that males grow faster and are more abundantthan females: this is so with P. kuhlii.

In species of the genus Sebastes (Love et al.,1990), it is observed in general, that these also havea low K, close to that of P. kuhlii, but on the otherhand, it is the females that reach a larger asymptoticsize. The K values estimated by other authors forSebastes spp. ranged between 0.04 and 0.30(Phillips, 1964; Lenarz, 1980; Archibald et al.,1981; Nelson and Quinn, 1986; Wilkins and Wein-berg, 1986 and Pearson et al., 1991). Although com-parison with other species of the same family doesnot suppose a conclusive confirmation of the resultsobtained, this still indicates that the values obtainedin the current study are not so far removed fromhypothetical values expected.

ACKNOWLEDGEMENTS

We would like to express our gratitude to all theparticipants of the project XIV DG 95/032, as wellas to our colleagues from the Centro Oceanográficode Canarias who support the sampling process car-ried out in this Centre.

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Scient. ed.: J. Lleonart


age and growth of pontinus kuhlii (bowdich, 1825) in the canary

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