## 2 Characteristics of Humpback Whale Songs  
The ocean is filled up of millions of sea creatures which emit sounds for detecting prey and predators, echo localizing and social interaction. Due to extremely low visibility in the sea, cetaceans rely on acoustic energy for communication [28]. The natural vocals produced by sea marine mammal ranges in the frequency band of 15 Hz to over 100 kHz [6]. They produce a variety of vocals termed as clicks, whistles, bursts and songs for different purposes [29]. They are recognized as sea noises as they are an unwanted signal for underwater communication. We take advantage of the naturally occurring sounds to embed information to realize covert mimicry communication.  
Humpback whales (_Megaptera novaeangliae_) are one of the biggest cetacean in the ocean. They produce clicks, whistles and songs with a high source level and low frequency range. Male whales are famous in producing long and loud songs lasting from several minutes to hours. If there is a song contest between cetaceans, a humpback whale song will no doubt be victorious [30]. As well as being aesthetically attractive, humpback whale songs have been and are still being studied extensively, mostly to understand why humpback whales sing. In fact only male humpbacks are known to sing and in general only when swimming alone, and songs are primarily performed in the winter breeding season [31].  
Generallyhumpback whale song lies in the frequency range of 8 Hz-8 kHz and the main energy concentrates on the bandwidth of 30 Hz-4 kHz. They emit songs at high source level around 175-188 dB re 1 fPa@lm (one femto Pascal measured at one meter distance) [32]. The song of a humpback whale can be heard from far distances in the ocean due to its high source level and low frequency range. The structure of a humpback whale song is the most complex vocal emitted by cetacean [33]. One whale sings continuously for hours. The song is composed of three to nine themes and it takes ten to fifteen minutes to return backto theinitial theme. Every theme compromises of phrases that repeat several times before a new theme. One phrase is made up of a series of different sound segments and lasts about several seconds. They also learn new songs and sing with new themes and patterns [34]. The pattern of a song is not fixed as it changes in different recordings. This is similar as a human singer sings multiple songs in various patterns and also learns and sings new songs [35]. However the humpback whale sings songs with a bandwidth of 8 kHz. The songs are broadband in nature modulated with a range of frequencies within the bandwidth. A sample structure of a complex humpback whale song phrase acquired from the ocean acoustic library [36] is shown in Figure 1.  
The song phrase has five unique segments. It can be clearly see from Figure 1 that all the song segments are frequency and amplitude modulated and differ from each other. This uniqueness in different segments is used as a benefit to perform covert operations by coding song segments into unique elements of bionic Morse codes. The communication signal will be composed of humpback whale song segments where the natural song segments will represent the information at the receiver. The eavesdropper will detect the signal but exclude it in a process of recognition as it will assume it as a natural humpback whale song giving perfect LPR constraint covert communication. We opted to mimic a humpback whale song for covert communication due to its characteristics. Long range transmission of a humpback song benefits us to do covert communication for a larger distance and large duration gives us an additional advantage to perform a covert operation for an elongated period.