Distinct Stocks of the Redtail Scad Decapterus kurroides Bleeker, 1855 (Perciformes: Carangidae) from the Northern Sulu and Southern Sibuyan Seas, Philippines Revealed from Otolith Morphometry and Shape Analysis

Abstract

A recent study was performed to assess the morphometric variation of otoliths of the Decapterus kurroides collected in the northern Sulu and southern Sibuyan seas in delineating fish stocks. Seven of the otolith morphometric descriptors (otolith length, OL; otolith height, OH; otolith weight OW; otolith area, OA; otolith perimeter, OP; ellipticity, EL; and aspect ratio, AR) demonstrated a significant positive correlation with fish length and six (rectangularity, RE; squareness, SQ; roundness, RO; circularity, CI; compactness, CO; and form factor, FF) demonstrated a significant negative correlation. In terms of intraspecific comparisons, almost all the otolith morphometric descriptors were significantly different between the two sites (except RE and OH). Further, principal component (PC) analysis showed that direct otolith morphometric descriptors such as OL, OH, OW, OA, and OP explained 61.71% of the differences (PC1). In contrast, derived otolith descriptors such as EL, AR, and CO explained 22.00% of the variations (PC2) for a total of 83.71% explained variations with the eight otolith morphometric descriptors. Statistics reveal that D. kurroides from the Sibuyan Sea have significantly larger, heavier, and more elliptical otoliths than those from the Sulu Sea. Results suggested that the D. kurroides from Sulu and Sibuyan seas are two different phenotypic stocks. Further studies such as otolith microchemistry, genetics, other life history-based studies, and present exploitation levels should be carried out to supplement the results of this study to fully establish the D. kurroides stock structures within Philippine waters.

1. Introduction

Otoliths are hard structures composed mainly of calcium carbonate situated in the membranous region of the inner ear of all teleost fish. These structures are primarily studied since they can provide a wide array of information on fish physiology, history, and environments such as growth characteristics, life history, migratory patterns, homing fidelity, microchemistry, and age data [1]. The potential application of fish otoliths to fishery science continues to advance and is being maximized by recently gaining the focus and attention of scientists [2]. Another factor strengthening the use of phenotypic markers such as otoliths is that phenotypic variations may exist even in the absence of genetic variation [3,4,5]. Fish otoliths are widely used for species and population segregation because of the variations in appearance and shape [6,7,8,9]. Further, otolith morphology was shown as a powerful indicator of fish stock and population structures [10,11,12,13,14,15,16]. For instance, otolith morphometrics and shape analysis were shown to discriminate stocks of several fish species such as Gadus morhua Linnaeus, 1758 [17], Glossogobius sparsipapillus Akihito and Meguro, 1976 [18], and Serranus cabrilla Linnaeus, 1758 [19].

Fish stocks, as defined by Hilborn and Walters [20], are a large group of fish that have similar life history traits and can independently sustain the population. Differences in distribution, life history, and genetics are a few of the key elements that identify fish stocks [21]. Certain manifestations of these include variations in morphology, genetics, movement patterns, maturity, growth, and other life traits [22]. Many works have shown that variation of fish otolith features (morphometry, shape, and microchemistry) of different stocks may be affected by several factors. Two of the main combined factors usually associated are environmental conditions (depth, salinity, and temperature) and genetics [2,8,23,24]. Other factors which may also affect otolith features are feeding habits and food availability [15,25,26]. The possible effects of fishing pressure in various fishing grounds on otolith shape and morphometry were also raised by various scientists [27,28,29,30].

The inherent characteristic of the Philippines as an archipelagic country with over 7000 islands that are separated by interconnected water bodies invites questions on the possible delineation of fish stocks. As these waters are constantly interacting with each other, the active exchange of fauna and nutrients that support them are certain. Additionally, the intrinsic characteristic of fish to migrate in various temporal and spatial scales needs to be factored in as this promotes mixing among separate populations. Treating separate stocks as one may lead to the localization of overfishing and management efforts. The fishing grounds within the Philippines are delineated into 12 Fisheries Management Areas (FMAs) for the purposes of resource management [31,32] but the scientific basis for the delineation of the boundaries of these areas to support fishery management initiatives is scanty. This study would provide a good opportunity for validating and possibly refining arbitrarily drawn boundaries of designated FMAs for managing fish stocks that are now largely under overfished conditions.

Information on the segregation of fish stocks in an archipelagic country like the Philippines is important, especially in studies on the assessment of stocks and their dynamics, thereby guiding management interventions that would promote the sustainable harvest of shared resources in an open-access fishing industry. The study of intraspecific phenotypic variations is essential to make strategies for sustainable harvest and conservation as it describes the flexibility of species in response to diverse habitats and the prevailing conditions therein [33,34,35,36].

The genus Decapterus is one of the most economically important fish groups in the Philippines that are widely distributed in various fishing grounds including the Bohol Sea (FMA 10), Davao Gulf (FMA 2), Moro Gulf (FMA 3), Sibuyan Sea (FMA 12), Sulu Sea (FMA 4), and Visayan Sea (FMA 11) [37,38]. At present, there are 11 species listed under the genus [39,40]. Species under the group are collectively locally known and reported as “galunggong” even in fishery statistics. Past and recent fishery production reports show that “galunggong” or roundscads are among the top commercially important small pelagic species in the Philippines, providing 4.24 to 4.59% (180,137.67 to 202,003.85 T) of the country’s total annual volume of fish production from 2020 to 2021 [41].

A subgroup in the genus is known as the redfin group clustered by Kimura et al. [39]. One member of this cluster is the redtail scad, Decapterus kurroides Bleeker, 1855, the species of interest in this study. The three other species are Decapterus akaadsi Abe, 1958, Decapterus tabl Berry, 1968, and Decapterus smithvanizi Kimura, Katahira and Kuriiwa, 2013 [42,43]. The D. kurroides is one of the main species caught in ring net and purse seine operations observed in selected fishing grounds such as Iligan Bay [44], Davao Gulf [45], Lingayen Gulf [46], and West Philippine Sea [37]. However, despite its economic importance, dominance in commercial gear catch, and ubiquity within Philippine waters, only a few studies were carried out to understand its biology, fishery, and stock dynamics.

At present, species delineation alone is difficult without the aid of molecular methods, which are expensive and time-consuming. Apparently, for this reason, the investigation of pelagic fish structures in the Philippines is rarely performed. In this study, we focus on two adjacent fishing grounds that are designated as FMA 4, represented herein by the Sulu Sea, and FMA 12, represented by the Sibuyan Sea. Both areas have basin-like characteristics and have limited water exchange with adjacent water bodies. The main goal of the study is to determine the possible separation of D. kurroides stocks using otoliths as this offers simpler and less expensive tools for fisheries management. Successful implementation of this investigation would offer a cheaper alternative for implementing population studies of other species and in other FMAs throughout the country and would provide inputs to support policies that are urgently needed for arresting declining fish populations in the central Philippines.

2. Materials and Methods

2.1. Sampling and Otolith Collection

A total of 170 samples of D. kurroides were collected from commercial catches landed in two fish landing sites of Panay Island (Miagao, Iloilo and Kalibo, Aklan), Philippines (Figure 1) from February to July 2021. Samples purchased from Miagao, Iloilo fish landing site represented Sulu Sea samples while samples from Kalibo, Aklan represented Sibuyan Sea samples. Both groups were fished using purse seines operated in the northern Sulu Sea under FMA 4 and southern Sibuyan Sea under FMA 12 based on preliminary surveys on fishers who operated the commercial fishing vessels. Fish were randomly sampled each month and were measured for total length (TL) to the nearest 0.1 cm and body weight (BW) to the nearest 0.1 g.

Sagittal otoliths were removed first by locating the D. kurroides otic capsule in the post-ventral portion of the neurocranium, anterior of the vertebrae. The otic capsule is then broken by creating a shallow incision in the middle and applying a gentle force at both ends. The fish sagittal otoliths were then collected and cleaned using distilled water, submerged in bleach for 20 to 30 s, air-dried, and stored in small plastic vials. For this study, only the right otoliths were processed for analysis. A total of 83 samples were used from the Sulu Sea and 87 samples from the Sibuyan Sea were used for morphometric analysis. Otoliths were laid on the non-sulcus side and images were taken using a stereomicroscope Olympus SZ61 (Evident Corporation, Tokyo, Japan) with a c-mount camera XCAM1080PHB (ToupTek Photonics, Hangzhou, China).

2.2. Otolith Morphometrics

Five size-related parameters were measured (otolith length, OL; otolith height, OH; otolith perimeter, OP; otolith area, OA; and otolith weight, OW) and eight parameters indicating shape indices (rectangularity, RE; squareness, SQ; ellipticity, EL; roundness, RO; aspect ratio, AR; form factor, FF; compactness, CO; and circularity, CI) were calculated. The OL, OH, OP, and OA were measured using Touptek ToupView 4.11 software (ToupTek Photonics, Hangzhou, China). Prior to measurement, calibration was performed to determine the number of pixels per 0.1 cm. The OW was determined using the analytical balance to the nearest 0.0001 g. Shape indices were calculated based on mathematical equations by [47] as presented in

Table 1. Otolith morphometric parameters and shape indices used in this study.

Size Parameters	Shape Indices	Equation
otolith length, OL	rectangularity, RE	$RE=\frac{OA}{OL\times OH}$
otolith height, OH	squareness, SQ	$SQ=\frac{OA}{OL\times OW}$
otolith perimeter, OP	ellipticity, EL	$EL=\frac{OL-OH}{OL+OH}$
otolith area, OA	roundness, RO	$RO=\frac{4OA}{\pi O{L}^2}$
otolith weight, OW	aspect ratio, AR	$AR=\frac{OL}{OH}$
	form factor, FF	$FF=\frac{4\pi OA}{O{P}^2}$
	compactness, CO	$CO=\frac{O{P}^2}{OA}$
	circularity, CI	$CI=\frac{OP}{O{A}^2}$

.

2.3. Otolith Shape Analysis

Two methods were used to describe the otoliths from the two populations of D. kurroides in the country. First, a few descriptions of the general features of the otoliths, specifically shape and outline, were carried out based on actual images. Terminologies followed [7]. The outlines were then detected using the ShapeR 0.1–5 [48] and Vegan 2.6-F4 [49] packages in R Studio v 4.4.1 [50]. Otolith images were first transformed into black-and-white to remove the effect of glares on the otolith surface (Figure 2), which affected outline detection in earlier attempts. Shape analysis was limited to outline detection, generating the Wavelet and Fourier coefficients, getting the measurements of otolith length, width, area, and perimeter, and applying Analysis of Variance (ANOVA) on shape descriptors using both Wavelet and Fourier coefficients.

2.4. Allometry Correction

For morphometric and shape studies, the individual size of the organism should be considered since it is the major source of variability [51]. For instance, fish otoliths are greatly influenced by fish size [52]. In this study, otolith morphometry and shape indices of D. kurroides were observed to have a strong correlation with fish size. To provide more valid comparisons, correction of data was carried out to remove the influence of allometry in the resulting otolith parameters. The equation used was based on the work of Deepa et al. [53] and is shown as:

$$M_s=M_o{\left(\frac{\underset{\_}{x}}{x}\right)}^b$$

(1)

where $M_s$ is the corrected otolith parameter; $M_o$ is the observed otolith size or shape parameter; $\underset{\_}{x}$ is the mean TL of all fish specimens in the group; and $x$ is the TL of the individual fish specimen. The slope b used in the equation was the slope of the linear regression between $log{M}_o$ and $logx$.

2.5. Independent Samples T-Test

To compare the values of measurements in the otoliths of fish and otolith samples from the two fishing grounds, an independent sample t-test was performed using IBM SPSS Statistics 27.0 (International Business Machines Corporation, New York, United States).

2.6. Principal Component Analysis

Each of the parameter’s mean was analyzed if there was a significant difference using independent sample t-tests at 0.05 level of significance. A Principal Component Analysis (PCA) was also carried out to identify the grouping of variables and to illustrate how the factors affect the grouping of samples within the component matrix. To ensure the appropriateness of PCA, the fundamental assumptions, such as multiple variable measurements at the continuous level, linear relationships between all variables, and sampling adequacy, were initially checked. Continuous level variable measurements are readily satisfied. However, since the units of measurement of the size parameters and shape indices are not all the same, standardization was performed using:

$$Z_{ijk}=\frac{X_{ijk}-{\underset{\_}{X}}_{jk}}{S_{jk}}$$

(2)

where Z_ijk is the standardized value of the ith observation of the jth variable at fishing ground k; X_ijk is the unstandardized value of the ith observation of the jth variable at fishing ground k; X_ijk is the unstandardized value of the ith observation of the jth variable at fishing ground k; $\underset{\_}{X}$_jk is the mean of the jth variable at fishing ground k; and S_jk is the standard deviation of the jth variable at fishing ground k. Linear relationships were confirmed using Pearson correlation coefficients. For sampling adequacy to ensure that PCA produces a reliable result, the Kaiser–Meyer–Olkin (KMO) test was computed. These statistical tests were carried out using IBM SPSS Statistics 27.0. PCA biplot was visualized using FactoMineR package in R v 4.2.1.

3. Results

Fish collected from the two sites were observed with significantly different total body sizes (p < 0.001) and weights (p < 0.001). Individuals obtained from the Sulu Sea ranged from 15.8 cm to 25.7 cm and weighed 44.8 g to 186 g, while they were 16.2 cm to 21.3 cm and 47.6 g to 109.9 g for the Sibuyan Sea, respectively. The average TL and BW of fish collected from the two seas are presented in

Table 2. Average total length and weight of fish collected from Sulu Sea and Sibuyan Sea, Philippines.

	n	Fish Length (cm)	Fish Weight (g)
Sulu Sea	83	17.9 ± 0.3	71.6 ± 3.8
Sibuyan Sea	87	19.5 ± 0.1	85.2 ± 1.1

.

3.1. Otolith Morphometry

Representative images of the right sagittal otoliths (non-sulcus side) for the Sulu Sea and Sibuyan Sea are presented in Figure 3. Using Pearson’s r correlation, it was revealed that the 13 descriptors had significant relationships (p < 0.01) with fish total length (

Table 3. Statistical relationships between D. kurroides otolith descriptors and fish total length.

Variable		Pearson Correlation	p-Value	Relationship to Fish Length
Direct otolith descriptors	OL	0.915 **	<0.001	Positive
	OH	0.847 **	<0.001	Positive
	OW	0.873 **	<0.001	Positive
	OA	0.917 **	<0.001	Positive
	OP	0.911 **	<0.001	Positive
Derived otolith descriptors	RE	–0.408 **	<0.001	Negative
	SQ	–0.832 **	<0.001	Negative
	EL	0.340 **	<0.001	Positive
	AR	0.347 **	<0.001	Positive
	RO	–0.496 **	<0.001	Negative
	CI	–0.879 **	<0.001	Negative
	CO	0.503 **	<0.001	Positive
	FF	–0.496 **	<0.001	Negative

** Correlation is significant at the 0.01 level (2-tailed).

). The five direct otolith descriptors (OL, OH, OW, OA, and OP) showed a significantly high positive correlation with fish total length. Among the eight derived otolith descriptors, on the other hand, EL, AR, and CO showed highly significant (p < 0.01) positive relationships with fish length while inverse relationships were observed between fish length and RE, SQ, RO, CI, and FF. These results are further visualized in Figure 4, wherein OL, OH, OW, OA, and OP can be seen in a cluster along the farthest right-hand side of the component 1 axis. The other derived descriptors, RO, FF, RE, SQ, and CI are on the left side of the axis with SQ and CI at the farthest left-hand side. This indicates the strong positive and negative relationships between the direct otolith descriptors from the derived ones, as described in Table 3.

Comparing the measurement values acquired from samples from the two fishing grounds, the independent sample t-tests revealed that 11 out of the 13 descriptors varied significantly. These were OL, OP, SQ, EL, AR, RO, CI, CO, FF (p < 0.01), OW, and OA (p < 0.05). Conversely, differences between OH and RE turned out to be indifferent (p > 0.05). Samples from the Sulu Sea were more square, rounder, more circular, and had a higher form factor while samples from the Sibuyan Sea were longer, wider, and heavier, and hence the area and perimeter were larger (

Table 4. Summary of independent sample t-tests on otolith descriptors of D. kurroides from Sulu and Sibuyan seas.

Variable		t	df	Sulu	Sibuyan	p-Value
Direct otolith descriptors	OL	–3.89	168	0.497	0.528	<0.001 **
	OH	–0.39	168	0.2217	0.2228	0.695
	OW	–2.14	168	0.0046	0.005	0.034 *
	OA	–2.57	168	0.0716	0.0766	0.011 *
	OP	–4.30	168	1.191	1.273	<0.001 **
Derived otolith descriptors	RE	–1.88	168	0.645	0.650	0.062
	SQ	4.55	168	32.67	29.37	<0.001 **
	EL	–7.55	168	0.380	0.410	<0.001 **
	AR	–7.32	168	2.240	2.370	<0.001 **
	RO	5.97	168	0.369	0.350	<0.001 **
	CI	3.70	168	246.82	221.6	<0.001 **
	CO	–7.02	168	19.940	21.210	<0.001 **
	FF	7.42	168	0.633	0.594	<0.001 **

** Significantly different at the 0.01 level (2-tailed) * Significantly different at the 0.05 level (2-tailed).

). Furthermore, values of rectangularity, ellipticity, aspect ratio, and compactness were higher in samples from the Sibuyan Sea.

Furthermore,

Table 5. Rotated component matrix extracted from all descriptors using Principal Component Analysis (PCA).

Variable		Component
		1	2
Direct otolith descriptors	OL	0.882	0.438
	OH	0.990	-
	OW	0.949	-
	OA	0.958	-
	OP	0.883	0.417
Derived otolith descriptors	RE	−0.538	-
	SQ	−0.855	−0.317
	EL	-	0.948
	AR	-	0.946
	RO	-	−0.878
	CI	−0.919	-
	CO	0.305	0.790
	FF	−0.300	−0.795

shows the grouping of otolith descriptors based on obtained values of various otolith parameter measurements from the two seas. This reveals that component 1 generally consisted of direct otolith descriptors with positive values ranging from 0.808 to 0.994, and some derived descriptors with small or negative values ranging from −0.30 to 0.3 (except EL, and AR). Meanwhile, component 2 was mainly composed of derived otolith descriptors with EL and AR as the highest value contributors with 0.968 and 0.965, respectively.

3.2. Principal Component Analysis

The attained KMO value is 0.8, showing that the sampling adequacy requirement is met (p < 0.01). Based on the grouping of the otolith descriptors, the summary of variance (

Table 6. Total variance of otolith descriptors based on component grouping of D. kurroides from Sulu and Sibuyan seas.

Component	Total	% of Variance	Cumulative %
1	8.023	61.71	61.71
2	2.860	22.0	83.71

) explained by the two-component groupings indicated that direct otolith descriptors (component 1) generally explained the 61.71% variation of the otoliths of the two D. kurroides stocks. Derived otolith descriptors (component 2) explained an additional 22.0% of the variation of the otolith parameters. These components, therefore, explained 83.71% of the total otolith variation between the two stocks of D. kurroides from the Sulu and Sibuyan seas.

In Figure 5, the grouping of D. kurroides otolith samples collected from the two seas is illustrated. Sulu Sea samples fell mostly on the negative part of the PC1 axis while Sibuyan Sea samples were slightly observed in the positive part of the axis. Moreover, Sulu Sea samples were also observed in the negative part of the PC2 axis while Sibuyan Sea samples were observed in the upper positive part of the PC2 axis.

3.3. Otolith Shape and Margin

Based on the metrics given by Tuset et al. [7], it was shown that the otoliths of D. kurroides from both the Sulu and Sibuyan seas were generally lanceolated in shape. The dorsal and ventral margins were sinuate and dentate, respectively. Additionally, as illustrated in Figure 3, the degree of dentation among the Sibuyan Sea otolith samples was more pronounced. Further, the anterior margin appeared to be peaked/pointed while the posterior margin was angled. Sulu Sea samples appeared to have a more prominent acute notch as well.

Using the ShapeR package by Libungan and Palsson [48], the mean right otolith shapes of D. kurroides from two populations were generated and are shown in Figure 6. Based on the figure alone, some parts of the otoliths were significantly distinct (p < 0.01) and were more pronounced along the dorsal region of the otoliths. For both populations, the antirostra were not prominent, hence the excisura ostii cannot be made out.

4. Discussion

In this study, morphometric and shape indices of otoliths were used to delineate stocks of D. kurroides from two adjacent fishing grounds in the Philippines. The D. kurroides from the Sibuyan Sea are longer and heavier than those collected from the Sulu Sea. Primary results have shown significant positive relationships between the fish length and the otolith descriptors. The positive relationships between fish length and the direct (OL, OH, OW, OA, and OP) and derived (EL, AR, and CO) descriptors are to be expected as these attributes (length, weight, and hence area and perimeter) go along with fish growth. From thereon, it can be drawn that with growth, the otolith becomes more elliptic, its aspect ratio increases, and it becomes more compact. Otoliths are known to grow continually subsequently with fish growth. New materials are deposited incrementally which contributes to the increase in values in these indices, while also incorporating materials that can be analyzed to trace back migration and environmental history (e.g., [54,55,56,57,58,59,60]). The negative relationships between fish length and the derived descriptors, RE, SQ, RO, and FF mean that as the fish grows, the rectangularity, circularity, and form factor decrease. This trend in the relationships between fish length and otolith size and shape descriptors was also observed in Merluccius capensis Castelnau, 1861 [61], Neogobius melanostomus (Pallas, 1814) [62,63], Decapterus macarellus (Cuvier, 1833) [64], Mulloidichthys flavolineatus (Lacepède, 1801) [47], and Terapon jarbua (Forsskål, 1775) [65]. Since otolith shapes are species-specific, it is not surprising that the relationship of the D. kurroides length can be directly or indirectly proportional to a specific otolith descriptor. Aside from being species-specific, otolith shapes can also vary from region to region, which may also be used to delineate fish stocks [6,66].

The independent sample t-tests revealed which descriptors separate the stocks from Sulu from those of Sibuyan. It was evident that D. kurroides from the Sibuyan Sea have longer, heavier, and wider otoliths than those from the Sulu Sea. In addition, the otoliths of D. kurroides from the Sulu Sea are less elliptical and less irregular than the Sibuyan Sea samples. These data are further elucidated in Figure 4 and Figure 5. Size-related indices (OL, OW, OA, and OP) were most important in distinguishing the otoliths of D. kurroides from the two fishing grounds. The observations in this study are consistent with the definition of Hilborn and Walters [20] for different stocks. Data on the mean shapes (Figure 6) of the otoliths have also shown clear regions of difference between the two populations.

Otolith shape is known to correspond to the distinct environment that the species is in [24], with genetics [67] or the interactions between the fish, environment, and its genetic make-up [68]. This then leads to the segregation of species into populations or stocks, sharing the same parameters for growth and mortality, as defined in Sparre and Venema [69]. In the case of D. kurroides from the seas of Sibuyan and Sulu, the populations may be treated as discrete based on the signatures derived from otoliths defined in shape and morphometry. It is, however, not proven or studied whether there is mixing between these two populations, which are in close proximity to each other (refer to Figure 1). Fish, especially pelagics, have an inherent characteristic to migrate in various spatial and temporal scales as a function of life history, foraging, and as an adaptive response.

A population study of Auxis thazard (Lacepède, 1800), Selar crumenophthalmus (Bloch, 1793), Rastrelliger kanagurta (Cuvier, 1816), and Sardinella lemuru Bleeker, 1853 collected from Celebes and Sulu seas did not reveal the occurrence of distinct stocks [70], and this is probably due to the effects on water exchange of the much larger Pacific Ocean to the Celebes Sea and of the West Philippine Sea on the Sulu Sea. Although our study is based on otoliths, the consistent difference of D. kurroides, which indicates the occurrence of different stocks based on the analyses of the samples themselves and their otoliths, is most likely due to the differences between the two water bodies. Although, the monthly sea surface temperatures between the Sulu (29.45 ± 0.71 °C) and Sibuyan (29.40 ± 0.95 °C) seas for the past decade are statistically similar [71] (p < 0.05), their sea surface salinity oscillates differently, due to their respective adjacent main water tributaries. Results from this previous study revealed that water salinity in the Sibuyan Sea is higher (>35 PSU) than in the Sulu Sea (<35 ppt), apparently because water exchange with adjacent water bodies is limited from the Pacific Ocean [72]. The salinity of the Sulu Sea is also affected by the less saline waters of the West Philippine Sea. These earlier results from others are worth reporting because salinity is one of the major factors that affects otolith variation among populations [5,57,73]. This is due to its ability to affect otolith’s aragonite development and element uptake [2]. These effects potentially cascade to several otolith features due to the variable formation of fish otoliths. It was also observed that fish exposed to more saline waters have longer [74], heavier otoliths with higher isotopic carbon and oxygen concentrations [75], and greater and wider otolith growth and increment depositions [76,77]. These may be attributed to the D. kurroides in the Sibuyan Sea having longer (more elliptic), larger, and heavier otoliths. Additionally, remote-sensed chlorophyll-a data from NASA [71] revealed that the Sibuyan Sea is more productive than the Sulu Sea, and this may also be a factor in the different otolith features and shapes between the two sites. Food availability potentially increases somatic growth, consequently increasing otolith growth [15,26]. In some cases, not only growth may be affected but also even otolith shape. Otolith shape analysis was successful at separating the otolith shapes of fish that experienced scarcity or an abundance of food [25]. This may have brought about the significant irregular (CO) and wider (AR) otoliths of Sibuyan Sea individuals.

In sum, this study demonstrates the application of otoliths as a tool for fishery management. It reveals the possible occurrence of different fish populations of D. kurroides in adjacent basin-like fishing grounds. It is necessary to consider investigating other sentinel pelagic species to confirm these results but the use of otoliths as a first recourse or in lieu of the more expensive molecular methods offers a powerful surrogate tool for generating inputs for fisheries policies at a much faster rate. This would undoubtedly help resolve species connectivity issues across the different FMAs and for managing critically threatened pelagic fisheries throughout the Philippines.

5. Conclusions

The use of fish otoliths to detect and delineate fish stocks is a practical tool that can be efficiently implemented due to its feasibility, sensitivity, and cost-effectiveness. In this study, an attempt to delineate fish stocks of D. kurroides from two neighboring fishing grounds in the Philippines was carried out by exploring variations in the morphometric characteristics and shape variation of D. kurroides otoliths. Based on the results, it was confirmed that the D. kurroides from the Sulu and Sibuyan seas are two different phenotypic stocks. Samples of D. kurroides from the Sibuyan Sea had significantly larger, heavier, more elliptical and irregular otoliths than those from the Sulu Sea. These were supported by the differences in the environmental parameters (salinity, water and nutrient circulation and distribution) in these fishing grounds, which have direct influences on otolith development and hence, morphometry and shape. Data generated and presented herein can be used for stock-specific management strategies that will promote the sustainable harvest of a common resource for the fishing provinces along the boundaries of the Sulu and Sibuyan seas. The same method can be utilized for studying a larger extent, i.e., the other nine FMAs with other commercially important species, to gain a better understanding of the dynamics of the different fish populations in the country.